Hedgewars - In this game, you control an army of hedgehogs bent on destroying all the other hedgehogs in your opponent's army. They get around via parachutes, ropes, or UFOs and have a huge trove of weapons at their disposal to off the enemy. Each play is timed so you don't overthink your next move, so just get out there and blow 'em up.
UFO: Alien Invasion - Your mission in this game is to defend the earth from marauding aliens bent on taking over the Earth. You'll need precision military skills and all your wits about you as you manage squads of armed troops, secure your bases and shoot down UFOs. You'll also have a cache of money to build, buy or produce anything your budget allows.
Frets On Fire - This fun guitar skill game kicked our own Kristin Shoemaker's butt earlier this year. The goal is to play the onscreen guitar as accurately as possible, using your keyboard. The game comes with a few songs included and you can also add your own. There are also four different challenge levels from basic to all the way up to "Kick Kristin's Butt."
The Battle for Wesnoth - This game is really popular with gamers who like fantasy-based themes. To regain the throne of Wesnoth that's rightfully yours, you must recruit and train a band of warriors to help you. There are more than 200 unit types in six factions, each with their own unique spells and fighting abilities. There are several multi-player options to choose from, including Internet play, so it's a great game for as many as eight players at a time.
Liquid War - This simple war game is unlike anything you've probably seen before and it's best described by the Web site itself: "Your army is a blob of liquid and you have to try and eat your opponents." Although you can play Liquid War alone, it was designed as a multi-player game to use with friends across a LAN or on the Internet.