I'm a huge fan of the Blueprint CSS framework. I've used it on two projects now and I'm really impressed. I really like the grid based layout and the cross-browser baseline provided by the reset.css file. There are some limitations/drawbacks. I've found that placing elements with pixel precision can be a pain, but not impossible. Also forgetting to setup the column classes correctly can result in some hard to find visual bugs. These problems are minor when compared to the benefits I've gained from using this framework.

Here is another CSS framework. I'm going to have to try one of these out soon.

Elements CSS Framework