I use Maxthon and Firefox. Firefox shouldn't need an introduction, but I can say a few words about Maxthon.
Maxthon is a browser which uses the IE core and basically improves IE quite a lot. It has tabbed browsing, search from the address bar, ad and pop-up blocking, URL aliases/nicknames, mouse gestures, easy page translation, and other things. It's very nice, but it's still IE, which means that it can display things strangly. Luckily the development is ongoning to have it run smoothly with the gecko engine as well.
The URL aliases is one thing that I really miss with Firefox, and I haven't been able to find out if there is a plugin/other solution for it. And I don't really feel like I have the time to dive into firefox plugin development and make it myself right now.
URL aliases let you set an alias so that you can (for instance) type "t17" and have the browser go to "http://forums.xisto.com/index.php;. It's very handy.
I also use Opera and Netscape from time to time to check that webpages that I (or someone else) made look okay with them as well.
Opera has let you switch user agents since the dawn of time. And if you use a browser which can't do that you could always use Proxomitron.