Mod Organizer (MO) is a tool for managing mod collections of arbitrary size. Supported games are currently Oblivion, Fallout 3, Fallout New Vegas and Skyrim.
It is specifically designed for people who like to experiment with mods and thus need an easy and reliable way to install and uninstall them.
Mod Organizer is the best utility to happen to game modding since the pencil and paper was invented! No more will you have to struggle with overwriting files as you incorrectly install mods. MO uses a 'virtual file system' that generates your modded game each time you load it. Simply download and install a mod, itself a painless operation with Nexus integration, then rearrange the order of each mod in the left hand pane until any potential conflicts are removed. MO then uses that list to generate a specific modded 'DATA' folder in the game you are running. (Skyrim, Oblivion, Fallout New Vegas or Fallout 3) Add to that the ability to have 'Profiles' and your gaming just became a whole lot easier, and better. Simply brilliant.