Magicka is a fun indy game I just bought. It has fun humor and a incredible spellcasting system.
You have 8 elements (with two you can create from the other 8) that you can combine up to 5 together and cast all of them 4 ways. Plus 21 (22 with the early bird special) spells you can collect.
I haven't tried multiplayer yet, but it looks fun and crazy.
And hey it's only 10 bucks. The only downside I really found is you can only save at the end of each chapter.