http://www.gamesas.com/topic/1338421-relz-wrye-bash/.
That and the Nexus Manager are, afaik, the ones that exist for Skyrim right now.
Which one's better - depends.
The Nexus Manager has the integration with the biggest mod site, and probably provides easier installation.
Wrye Bash is stronger when it comes to managing mods - solving conflicts, updating savegames, rollback of mods, that kind of things.