Forge Modloader FML is most used in minecraft servers. Firstly, it was a Mod Loading System for minecraft servers, later it was upgraded and now it is available for bukkit server and now this mod is also available for clients. The Forge Modloader FML can now be used without Minecraft Forge both on the client and server side with full modloading capability. It is fully compatible with all modloader based mods.
This must be the best mod loading system / modloader ever! If you need some more info, check the readme text file located in mod’s .zip archive file. It will definitely help you!