Twitter Bootstrap has a "responsive" grid system (described here). An over-simplification is that it adapts to lower resolution screens by falling back from horizontal components to stacked components. I've found the CSS framework to be pretty easy to use, and with just enough features to keep me going without bogging me down with a telephone-book sized tome of documentation. The docs are good, and wherever they fall short I'm able to just "view source" on the documentation pages themselves to see how some effect is created. And there are a bunch of jQuery-based JavaScript plugins for it.

It sure beats the old days of hand-rolling (poorly) this stuff myself.