I mainly use.
To Many Items: Easier to use than the standard creative mode. You can also switch from survival to creative mode along with turning off/on rain and day/night.
Singleplayer Commands: Only like it because of world edit.
Dynamic Height: You can set your worlds height to 32, 64, 128, 512, 1024, and 2048. I love playing on 512.
Mo' Creatures: More creatures

. Favorite would be breading horses and flying them around.
Reis Minimap: Better than Zains Minimp in every way. Mainly use it to find chuncks and slime chuncks for better Mob Towers. Very useful for marking places on the minimap.
Millénaire: Very fun to have when you have Mo' Creatuers. Also fun to have it with Aether and have an Aether village.
Im also to lazy to build my own house sometimes so i just buy one

.
Aether: Very fun and dangerous place. Its like a whole new minecraft. I love making minecart tracks/stations here.
EDIT:
Cant forget about
MC Patcher: Cant go wrong with HD 256x256 texture packs.