Firefox 4 Beta
The main reason that I'm addicted to Firefox are the plugins; as much as I like Chrome and Safari they just don't have the vibrant plugin community that Mozilla has.
One of the big complaints against Firefox over the years has been memory usage. But since Firefox now includes process isolation for plugins, one can see who's to blame for using up all your system's RAM. On my system the answer seems invariably to be Flash.
The Firefox 4 beta, which I've been using for quite a while now, includes this feature-- and it's definitely worth checking out. Be sure to look at how much memory your Flash plugin uses, and also keep an eye on how often the plugin crashes. You might be surprised just how bad it is.