http://duelmail.thewitcher.com/
A simple little game that has me hooked.
It's in free open beta right now.
I don't know if I would pay to play it, once the beta ends, but I'm enjoying it right now.
No need to download anything, it plays in your browser using Flash. Sign up, choose a character type. Fight.
The fights play out like extended rock-paper-scissors at first, but as your character levels, it starts to become more and more complex.
If anyone joins, Evilturnip the Sorceress (I thought I would be able to use a different name for my character, but it uses whatever you choose as your login) will take on any comers!