IE6+, FF2+, Chrome, Safari 3.x+, Opera 9+
That covers 99% of traffic in my experience, and does well in any browsers not on that list too (due to the range of browser engines it does definitely work on)
At first I sucked at making CSS+XHTML sites work well on all browsers, but nowadays my sites tend to work fine on all the above browsers, with only a few IE-specific CSS rules.