Quadradius is a game that can be played for free online, with no registration or anything, that plays somewhat like checkers, only with power-ups that your pieces can pickup to do various actions. there's a brief tutorial/explanation on the site. Being a member allows you access to different sized games and in member vs. member matches, cool new/different powers, that are added to every month or two.
So I enjoy it, my member name is Salmon, my guest name: Kevin. Hope to see you there!