When I'm on Windows I always use Firefox, mainly because like the others said the amount of extensions that are created. I would use Opera but I just don't like the style it renders things in, besides it doesn't have extensions to make life easier. Although Firefox might run on a XUL based engine which might take up a bit more memory space, I don't really mind. IE and IE7 are definitely what I am going to use right now. Mainly because IE 6 is too slow and IE 7 still has a long way to go in terms of compatibility and how it renders things.

While I'm on my mac it's a different story... If I'm writing a post or perhaps writing an e-mail or something like that I usually prefer Safari because of the in built spell check it has. When I'm just browsing around I prefer Firefox or Camino (well Camino is designed for OS X so it just makes the browsing experience all the more better, not to mention the GUI). So it's sort of like a three way tie between Safari, Firefox and Camino, depending on what I want to do...