This is a small piece I did showcasing 4 giants in minecraft. I did not discover these myself, but some of them are less well known, so I thought I'd showcase a few of them. Most of them people know about, but don't know how to make, In wich case feel free to look at the commands in the description below! 

Giants include:

Regular Giant (Giant Zombie)
This Giant is probably the most well known. This one is quite simple, and has been programmed into the game for quite a while now as a vanilla mob, but hasn't yet been implemented and doesn't naturally spawn
Command:
/summon Giant ~ ~2 ~5

Giant Slime/Magma Cube
This is a giant slime or magma cube that is not quite as simple to spawn, but is still fairly easy! So far, I haven' t found a way to damage them without commands, but I'm not sure if it's possible or not. This command takes advantage of the fact that with /summon Slime, you are allowed to modify the size of the slime to your heart's content. Fortunately for us, there is no limit on this size, allowing us to make giant versions of our slimey friends!
Command:
/summon Slime ~ ~2 ~5 {Size:100}
/summon LavaSlime ~ ~2 ~5 {Size:100}

Giant Wither Boss
This one is interesting. Yet a little harder to spawn, but still not too difficult. You may notice that immidiatly after a wither is spawned, during it's brief invulnerability phase, it is slightly smaller than usual. We are allowed to change just How much smaller it is. In fact, we can make it so much smaller that it inverts on itself, turning upside down. And it keeps growing after it inverts. With this command, we force it to invert to the point were it grows to collosal size, than we give it the name Grumm (or Dinnerbone if you prefer) to turn it right side up again. Also, with this command, the wither is not blue during invulnerability and stays this big permenently. I'm fairly certian that this one can't be damaged without using @e that comes with the new snapshots.
Command:
/summon WitherBoss ~ ~2 ~5 {Invul:100000,CustomName:Grumm}