This report shows statistics aggregated across all active websites tracked by W3Counter.
For example, 11% of all viewers still look at the web at 800x600. That is a lot, considering the stats are gathered based on an analysis of 1,536,728 distinct visits across 1,507 websites (today).
Users of Firefox 1.0 and 1.5 make up 26% of viewers. That is an encouraging number for web developers everywhere.
They even offer stat tracking for your own site. It costs money, of course, but it looks like a good service if you do not have a stats package on your web server.