Absorption

One of the more powerful foes faced by the Z soldiers was capable of gaining huge power ups via absorption. Cell was his name, and he was a collection of DNA from the best warriors in the galaxy. He had a tail which allowed him to absorb people, and the androids, which made him unbelievably powerful in comparison to everyone else. The increases that he gains rivals that of Super Saiyans. If he is hit hard enough though, he will regurgitate a product of the absorption, such as an android, and become less powerful. Cell has 3 forms, reaching the perfect form by absorbing #18.

Guru Charge-Up

This is a major power-up gained by only the most noble warriors, with a pure heart, and good intentions. Guru releases the hidden potential inside the person he is doing it on, giving them an extra power, like more strength, or healing capabilities if they had the potential to do that. He does this on Gohan and Krillen and gives them huge strength boosts, but when he does it on Dende, he gives him the aforementioned healing power.

Kai-O-Ken  

One of Goku's most useful allies, an awesome attack taught to him by the martial arts master King Kai. It increases everything massively, including his speed, reflexes, even sight and hearing. The user is surrounded in a red aura, which is so intense that it destroys everything around them, and even begins to rip into their clothes. The major disadvantage is that even a Saiyans body can't sustain very much of it, and will explode if any attempts are made at a level of Kai-o-ken which is too much for the level of the user. Even with this major weakness, the kai-o-ken was a huge ally all the way through the DBZ series, with Goku using it a lot in the Earth-saving battles he had. 

Ki Absorption

This is an extremely useful technique used by the androids where they absorb the energy from any living thing. For instance, they could grab a powerful fighter, and drain him of his incredibly strength, being given it as his own. Dr Gero and #19 are the ones who perform these moves, with Gero physically grabbing the enemy and literally sucking the life out of them with his hands. #19 has a similar technique too. Another pro of the energy absorption is that it can be done against ki attacks. An example of this is when #19 used a device in his hand to suck in Goku's KameHameHa, and use it to boost his power greatly. Gero could also do this, making them both very powerful opponents, with limitless potential.

Machio Star Power Up

In the Garlic Jr. saga, there was a small, red planet called the Machio star, which gave huge power ups to any evil-doers, allowing them to become infinitely more powerful. This is an incredibly useful thing, which can give great tactical advantages, especially when it gets close to Earth, making the evil people even stronger, able to transform into something similar to ascended Saiyan, with huge muscles, and loads of power.

Namekian Fusion

The fusion between Nameks is slightly different to the other kinds. Rather than merging into one completely new being, Nameks fuse into one another, with one being the base, and one submitting their power and knowledge into that person. An example of this is when Piccolo fused with Kami, he was given his wise knowledge, and extra strength, but still had complete control over his thoughts and actions. 

From what has been said by people such as Piccolo and Kami, it seems that it is possible for the Nameks to split at anytime they feel like, although they never do. It seems as though a slight appearance change is apparent, with their facial features shifting slightly, so that it is almost unnoticeable. Anyone familiar with 'Kamiccolo' would have noticed this. 

For the actual fusion to start, the base Namek puts his hand on the other Namek, and they fuse into one, with the body of the Namek submitting his power disappearing. This method has it's strengths and weaknesses, but it's main ones is that it lasts a lot longer, albeit not making the product of the fuse as strong as in the Saiyan one.

Oozaru

When a Saiyan looks up at a full moon, and has a tail, then a strange transformation occurs. They become even more violent, and most lose control of their actions, becoming a terrible destructive force. Vegeta is the only know person who can control themselves after transforming into an Oozaru. The biggest strength of turning into one of these giant ape-like creatures is that your fighting power is increased 10 fold, and you are even faster, despite your intimidating size. Without a tail or the moon, a Saiyan cannot maintain, or turn into their Oozaru state, meaning they are degraded back into their normal form, losing a lot of energy in doing so.

Saiyan Fusion

This technique makes the users far stronger, as they fuse temporarily into one being. It only works if the two people are at around the same strength when reenacting the dance of wearing the earrings. Fusion can be accomplished in one of two ways, which are the aforementioned dance and earring methods.

With the Pottara earrings, the two wearers must put on on their corresponding ear to their counterpart. They will then automatically be fused into one until they remove the earrings. Goku and Vegeta used this technique to get inside Buu, but Vegeta crushed his earring when they had separated, leaving no hope of fusing by that method.

The fusion dance is a lot less complex, and it involves the people involved doing a short, symmetrical sequence of movements, rather like a dance, thus the name. They then join forefingers, and merge into one. This lasts for a certain amount of time, varying on the strength of the people who participated in the dance. Usually it is only about 20 minutes though. As with the earrings, the two Saiyans must be at approximately the same strength for it work correctly. Of course, as with anything, mistakes can be made, with devastating consequences. Many a time has come when people have messed up, and created a fat, or thin entity not even close to having the power as they had alone. Fat Gogetta is an example of this, but when it works, the fusion can be deadly, as Super Saiyan Gogetta proved. Gotenks was the only fused Saiyan to reach SSJ3.

Super Android

Not many people are aware of this transformation, as it only happens in a film, and in Dragonball GT. It happens from a technique which is extremely similar to fusion, where the android absorbs parts of other androids in order to gain a power up and become even more of a monster. Because of the way Dr Gero built #13, he was able to use the parts of the other dead androids to make him far stronger, turning into an almost unstoppable force, far stronger than even a Super Saiyan. There doesn't seem to be any downside to this form either, other than a sudden uncontrollable urge for violence, but as it isn't a very likely occurrence for androids not to be violent, it doesn't really matter. The first Super Androids appeared in Movie 7: The Three Super Saiyan, and as already mentioned, it was #13.

Super Saiyan

The Super Saiyan is easily the most popular power up, or indeed transformation, made famous by Goku back in the Namek saga. A legend until then, it was unheard of that anyone could ever possibly possess the power of a Super Saiyan, which is why Frieza destroyed the planet Vegeta, he was scared, and for good reason too. When a Saiyan transforms into this state, their eyes turn green, and their hair sticks up and goes blonde. The power increase they receive really is immense, possibly the biggest of any transformation in Dragonball Z. 

To become a Super Saiyan, you must first have a pure heart, either a goodly pure, or a pure evil one. Secondly you must have already achieved incredible power, superior to that of almost everyone else's. The final step is overwhelming anger, not usually possessed until a loved one is killed, or a series of terrible events happen. When the transformation occurs though, the Saiyans go ballistic, and fight with anger in their heart. With time, the level of Super Saiyan (Or SSJ as it is commonly referred to as) can be controlled, and the anger in your heart can be diminished. An interesting fact is that you cannot form a Spirit Bomb whilst you are SSJ, due to the rage being too much. Goku was the first ever Super Saiyan, and it can be debated as to whether Trunks or Vegeta was second, as Trunks was second one seen, but in his timeline he would have been forth had the events gone the same as the real one.

Size Transformation

Some people, mainly Nameks, can increase their size and power. This isn't really in the same category as the traditional transformation that it mentioned below, as the appearance of the caster never changes, only the size does. Piccolo uses this against Goku in the late stages of Dragonball, and against Garlic Jr. about midway through Dragonball Z. It makes them stronger, but a little sluggish, which is why it isn't utilized very often. It can also expose many vulnerable weaknesses.

Transformation

Some warriors have the ability to condense their power in weaker forms, as their real appearance has power that is too radical to control. People with this ability can change at will, and it varies from being a lengthy transformation, to a very quick, somewhat hasty one. Zarbon and Frieza both had this power, and they utilized it to its full extent before eventually being defeated. When at their most powerful form, it is assumed that the person in control can demote their power back down to their energy conserving form, but there is nothing to prove it in Frieza's case. 

Some of the transformations that occur give only a minor power boost, whilst others increase every single factor significantly, such as speed, strength, power, etc.