World of Goo Puzzle Game
World of Goo is a wonderfully crafted puzzle game by 2D Boy, an independent game developer consisting of Kyle Gabler and Ron Carmel, both former EA employees. Watch this beautiful World of Goo video trailer(love its sound score!).
World of Goo costs US$19.95 apiece. There is a demo version of the game available for free to download and install. More info here.
Vendetta Online
Vendetta Online is a 3D space combat MMORPG. This MMO permits thousands of players to interact as the pilots of spaceships in a vast universe. Users may build their characters in any direction they desire, becoming rich captains of industry, military heroes, or outlaws.
Though Vendetta Online is available in the paid software section of Ubuntu Software Center, no price is listed yet. You can always download the demo version of the game from here.