Playing the game through Steam? It's integrated through Steam Workshop. Want to play a multiplayer game with your favorite mod? There's full multiplayer support and the ability to see which mods are enabled in a given game. Want to start modding but have no idea where to start? There's a whole wiki filled with documentation on how to get started.
Don't own the game but love the idea of modding a multiplayer game? That's cool, too. The game is on sale until April 8th for $9.99 to celebrate the release of the modding tools. So if this sort of thing tickles your fancy, you've got almost no reason not to check it out.