Desktop Tower Defense is a Flash game that has simple rules but is great fun and very addictive. The bad guys enter from the top and left and try to cross over the desk to the other side. You loose one of your twenty lives for each bad guy that makes it over.
My only complaint is that it does not work on the Wii
P.S. Use your resources to upgrade a few central pieces to the max firepower like in the example maze above.