For the Fantasy genre, I'd recommend Hârn. It can be played with any system, but it does have a customized system that works quite well. No levels, no hit points. Fairly gritty feel.
http://www.columbiagames.com/harn/
Plus, there's a ton of free stuff available for it at http://www.lythia.com/ (including one of my own: http://www.lythia.com/2010/02/bognors-folly/).

For the superhero genre, it's hard to go wrong with Champions/HERO system, IMO, but I guess that may depend a lot on the specific likes/dislikes of your group.