Valve previously announced that it would eventually allow third party content creators to sell their creations through the Steam Workshop. This has previously been running with Team Fortress 2 with players creating and selling hats.
Now though, community based content creators can sell mods for games which have enabled the feature, the first game to see the feature in use and allow content to be sold for it is Skyrim. Already with a large modding community behind it, Skyrim will allow those mods to be exchanged for financial incentive beyond donations the modding team may have been collecting previously.
Charging money for mods is entirely optional and can still be released free to players but it seems a number of mods (19 so far) have opted in which you can browse on the Steam site. Some mods have not taken up the offer (yet) like popular mod, Falskaar. Falskaar is currently still available free of charge and boasts 20-30 hours of additional gameplay.