On: CSS Framework for Dummies

[Liblor]

Looks interesting especially for non-designers like me. I also recommend Skeleton[1] for a simple and clean look. It has some additional features and makes use of classes, but is still quite simple.

[1] http://getskeleton.com/

[greenjellybean → Liblor]

I love skeleton and use it almost exclusively. Most of the classes are simple and you can easily overwrite stuff to make the design unique.