Right now at GamersGate you can get Mount & Blade: Warband for $9.98 (50% off). (The entire series is on-sale, but this one is the only one really worth picking up.)
I highly recommend this game. The mods for it are outstanding as well.
Also want to mention, this game has a pretty steep learning curve, but once you get the hang of it you will have a blast. (If medieval fighting and conquering the world allures you.)