DragonAPI by Reika
DragonAPI is a function library and support mod which is relied upon by all of my mods. If you do not have the API, you will crash on game load with a NoClassDefFoundError mentioning DragonAPIMod.class
Enchiridion by joshiejack
This is a mod that started off as part of Mariculture, allowing for loading of custom books from zip files in the configs, handy for modpack creators or people who just want to let people know what's going on, with a mod.
Forge Mod Loader by cpw, LexManos
The Forge Mod Loader provides the ability for systems to load mods from the file system. It also provides key capabilities for mods to be able to cooperate and provide a good modding environment. The mod loading system is compatible with ModLoader, all your ModLoader mods should work.
Forge Multipart by ChickenBones
Open source library for facilitation of multiple functional parts in the one block space.
HeldCore by heldplayer
Just some utilities I use when making mods, makes it easier for me so I don't have to re-distribute them all the time.