Team Fortress 2 is still chugging in a big way, and the latest bit of official content to drop is the Mann Co. maps, which are now available by way of Early Access. You can volunteer to test the maps for Valve by clicking the Play Multiplayer option, then enabling the Play Beta Maps box.
Basically, these are new maps that allow you test out a whole bunch of new deadly mechanics. Valve notes that simply playing them and keeping the servers full will help in the developmental process.
Mann Co. Beta Maps - Early Access [Team Fortress]
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12:30 PM on 12.19.2014