I've been using Nicole Sullivan's "Object Oriented" CSS grid for a while now (hate the term, though) and have found it to be quite good. However, as I get more experience with grids, I'm noticing a lot of other frameworks out there; in particular the 960 Grid System and the Yahoo! UI Library.
I'm looking for other peoples' experiences with these alternatives (or others) so I can make a more well-founded decision on whether to change or not.
I think Blueprint is the best.
Also review comments posted at: http://stackoverflow.com/questions/589184/help-me-choose-a-css-framework-960-vs-blueprint-vs
I found the noupe site has great comparisons for CSS frameworks and helped me choose: http://www.noupe.com/css/5-popular-css-frameworks-tutorials-tools-for-getting-started.html
If you are using rails framework, you should consider using compass which is layer over these frameworks (As suggested by Alan below).
I would recommend Blueprint used from within Compass. Compass reduces redundant CSS styles and has variables and classes. Apart from that, I have found Blueprint to be easy to use.