I use 5 different browsers approximately as:
70% - Flock - This is like a Firefox on steroids (actually based on the Firefox engine), allows me to blog from it, integrates with flickr, etc, Any firefox extension that I use, I load on this browser
10% - Firefox - I use this browser pretty much clean of all extensions, when I want to load a site and leave it running on the background I use this browser.
10% - Internet Explorer 7 - Beta 3 at this point, so far I like it a lot, but there are a bunch of sites that just refuse to work with it, once it becomes a release version I'm sure I'll use it more
5% - Internet Explorer 6 - On the other hand, there are some sites that only look good or only allow Internet Explorer 6, I use this browser for IE specific sites and/or sites that I trust (e.g. intranet)
5% - Opera - Since out of my list of browsers, this is the least used, this kinda gives me more "protection" when I'm visiting sites that I don't trust that much (e.g. sites about hacking)