FPS Game: Dev Test is a First person shooter that is strictly built of a development asset. Its intended purpose is to test and showcase networking features and other advanced Steam systems. It will continue to adapt and have new mechanics and testable features as time goes on. It is intended for multiplayer PVP combat and sessions, but creates online sessions using listen server at the moment. This is good if you want to just play with your friends only in a small private game.
You can play single player against bots, or multiplayer (can include bots as well). It uses a server browser, so look for a game and join random people. or create a game for you and your friends for the private experience if you'd prefer.