I use Choosy so I can switch between browsers easily. Instead of having to set Safari, Firefox, Google Chrome, or some other browser as my default, if I have one browser open, links will automatically open that browser. If more than one browser is open, Choosy will let you pick which one to open a link in.
Today I discovered that it’s even more awesome than I realized: It not only lets you choose Mac OS X browsers — it also supports Windows browsers running under VMWare Fusion 3 (I haven’t tried it with Parallels yet, but I suspect it will work as well). If you have both a Windows and Mac OS X browser open, Choosy will show both of them in its menu and let you open links in either one.