Firefox + adblock + flashblock + scriptblock + pop-upblock + ... (yep, I'm paranoid. I figure if something there's something that I need to run pop-ups on, for instance, I can just make an "exception allow." Currently, the only sites I have to do that for are PayPal and USPS - both for the online shipping labels. Which come up in Acrobat under FireFox just fine...)
MSIE? Only if I can't run the site any other way (if that's so, I also write the webmaster and tell them they need to fix that little flaw. If you can only use the site in a single browser, it's designed badly.)
I've got FireFox, old versions of Netscape, old versions of Opera (I tried Safari, but think it's a resource hog,) and I may even have an old version or two of Lynx floating around here somewhere.
MSIE? Fscking joke. Merci, non, monsuier Murphy.