LiteLoader

LiteLoader is a mod API created by Mumfrey. It is designed for mods that change various aspects of the client, such as VoxelMenu. LiteLoader was primarily made for the Voxel Modpack Team, but is free for anyone to use. This API cannot be used by modders to add new blocks, items, mechanics, or creatures into the game, Minecraft Forge must be used for that. This is due to LiteLoader being a client-side API, whereas Minecraft Forge is server-side and client-side.

Unlike most mods, this mod has a Windows version, and a UNIX/Mac version. The installation process can be found on the Minecraft Forum thread.