On my Mac, I use Firefox 99% of the time. Then, in decreasing order of usage, it's probably Opera 10, then Safari. I also download Camino from time to time just to see how it's doing but I rarely use it.
On my PC, I also use Firefox about 99% of the time. Then, in decreasing order, probably Opera 10, Internet Explorer 8, Chrome, and Safari. However, I rarely use Safari on Windows except to see how the latest version on Windows is doing (just so I know).
I like to keep various browsers around for two purposes. First is testing Web pages I develop with the "big four" rendering engines (Gecko, Trident, WebKit, and Presto). However, it's also nice to open a rarely-used browser to, for example, log into a website with a different (such as Gmail) with a different account from your normal one or when other people want to use your computer quick (although I'd never give them IE for that purpose 😀).