There's 3 mods that are working on making Skyrim multiplayer.
I know Skyrim Online works currently, though obviously is lacking some features currently.
Tytanis is working on bringing together several multiplayer mods. So far there is a functioning multiplayer function running on a single server, but currently the only thing you can do is walk around and jump. You and all other players are locked in a 'T' pose that is the default for all player models.