Despite being part of the recent paid-for-mods fiasco that Valve launched, Skyrim has been one of the most heavily modded games in recent memory, keeping it alive almost four years after release. That shows no sign of stopping in the near future either, as a new mod may open up all sorts of gameplay potential from the open world RPG.
It's called Tamriel online and it adds coop play to Skyrim, though not in a fashion pushed by other mods from the past which have opened it up for plenty of players to quest around together. This one is just focused on making it so you and a buddy can romp around the world and... that's about it for now.
Unfortunately being as early access as it is, this mod doesn't have combat - only one player can see it - and quests don't work either. So really it's only good for wandering around in at the moment, but it does mean players can explore the world and discover some of its secrets.
Even if they can't really use them.
The next step for the one man developer team is to work on squashing any obvious bugs, before moving on to adding features back to the game so there is a little more for you and your pals to do.