Two quality Tripwire Interactive games -- Killing Floor and Red Orchestra 2 -- are getting some modding love. That is to say, Killing Floor now has Steam Workshop support, and Red Orchestra 2 will "very soon" as well.
Steam Workshop, if you still don't know, seems to be the easiest ways these days of finding, tracking, and installing mods, particularly if you already buy everything on Steam anyway. See here for what it does with Killing Floor.
Also of note: all Tripwire games are discounted now on Steam for some undetermined amount of time.