"Most 64-bit Linux distros come with a 64-bit version of Firefox. There is also one of those for Windows. Other than that, I don't think there are many 64-bit browsers.
To be honest, you don't really need one. The speed increase would be marginal at best, and most plugins (Flash) aren't 64-bit-compatible yet (although Java finally made a 64-bit version of theirs). You'd gain nothing from using a 64-bit browser, so there's no point. Developers don't want to waste the time, either, so you probably won't see too many in the near future (at least not for Windows)."