SDK's Gun Mod- COMPLETE in balance of field here, but still cool if you're a gun buff like me.
Planes v2- Very cool way to explore a world quickly, can be a little laggy on slower machines due to chunk generation and the planes are a little hard to control in terms of altitude.
Poke'ball- Very cool, balanced, compatible with most other mods, love it. I CHOOSE YOU, CREEPER!
HumanPlus- My favorite, adds a whole new level of life to Minecraft. They talk, walk and stalk. I find myself building stuff to impress the AI.
"Feel my blade, Creeper!"