In September, Team Fortress 2 modders introduced a new charity operation. Players are invited to go to the PvE mode Mann versus Machine to fight waves of robots:
Reload your weapons, prepare your explosives, fill your flasks, and stock up on upgrades, because in this war, Gray Mann is showing himself like never before — this could be the last war if you don't prepare properly...
The operation was named "Operation Lithium Lunacy". For Team Fortress 2, a set of 9 medium-level missions, 9 advanced, 2 "nightmare" difficulty, and 10 "incredibly difficult" missions were prepared. To receive virtual medals, at least six missions must be completed. Modders also provided several special Master-level missions for hardcore enthusiasts.
A list of community servers with Operation Lithium Lunacy missions can be found at the following link.
Valve itself is also preparing new content for the Mann versus Machine mode, but Team Fortress 2 developers have not disclosed details of the planned update. The release date or release window has not yet been announced either.