Running with Rifles is a game that is perhaps a little too old school in its ideology. The method by which you have to setup a co-op match alone makes me feel like I should be using a 56k modem. Anton penned our written co-op review, but he also shared a few thoughts in video format, including a helpful "how to setup a co-op match" section.
You can check out a demo of Running with Rifles on Steam before deciding whether or not to take the $15 plunge for the full game.