I was persuaded to buy a game on Steam called Magicka earlier this week and I have to say it's pretty good considering the price. It's a third person top down RPG with 4 player co-op, but what makes this game super sweet is the spell casting system.
The player has 8 basic elements to cast spells with (water, life, protection, frost, lightning, arcane, earth and fire) with which he/she can press combinations to cast a huge variety of spells. Each spell combination has a targeted version and an area of effect version and some elements combine into others (water + frost = ice). The co-op allows players to combine beams for extra power and beams containing opposing elements crossing cause explosions. On top of this you can unlock special spells throughout the game like teleport and raise dead.
A personal favourite of mine is to soak all enemies with a water AOE and then use a lightning-frost spell to chain-freeze them all in place. The game is a lot of fun and I still don't think i've seen all the spell combinations yet. It's definitely worth checking out.