This weekend until 2:00PM Eastern on Sunday you can play an awesome first-person medieval battle game Chivalry: Medieval Warfare for free. Head on over to the Steam page to download it and start playing. If you decide you like it, you can then buy the title for only $8.49 (£6.45 in the UK), which is a heck of deal.
Fraser really dug it in his review, and I am also a big fan. The story behind it is really cool too. It started as a bunch of guys working together online to make a Half-Life mod, and now they have a full-blown retail game. Very cool.