1. Finding Browser share
When you want to support most users, you have to try to figure out what they’re using. There are at least three or four different sites which give you their browser share, but I think you have to pay if you want really accurate, detailed information; here’s one source, here’s another, and here’s one last site. Update, 11/3: here are stats for the www.bbc.co.uk homepage.
3. Getting ahold of old browsers and older operating systems, so you can test
Luckily, this is fairly easy to solve. VMWare (which I’ve written about previously) takes care of the various operating systems (well, that and a mac mini) that we need to test under. And a simple google search turned up a fantastic archive of old browsers: browsers.evolt.org, which has many different browsers going all the way back to NCSA Mosaic!.