Croteam, the developers behind Serious Sam 3 announced today the addition of Steam Workshop support for their game and mod tools.
After a brief beta period, the Serious Sam 3 Steam Workshop launches with over 60 mods available for immediate download, created by talented community modders and mappers. The custom content includes multiplayer and survival mode maps, multiplayer character skins, weapon modifications/re-skins, enemy modifications/re-skins, gameplay changes to campaign mode, and much more.
Simply browse around the Workshop and subscribe to any mods, maps, or items you wish to add to your game. Be sure to check the tags and description to understand what game modes the item is available in. Then, the next time you start your copy of the game the items will be downloaded (or updated) and be ready for use.
If you're interested in trying your own hand at modding Serious Sam 3, you can check out their wiki here[serioussam.com] to get started. The editor itself is already listed in your Steam tools library if you own the game--select "Tools" from the drop down in your Steam library and look for "Serious Sam 3 Editor".