If you want something that can be fairly addicting, I want to recommend Meth. If you have some sort of anti-drug blah blah policy, then try Stumbleupon. It is toolbar based, which I am not a fan of. It installs in IE and they do make a Firefox one. Since this is toolbar based (Have I mentioned I am not a fan of toolbars?) I installed it on my work computer IE. You "stumble" based upon what interests you have indicated you want to stumble upon. I am currently fixated on blogs.
I am a Firefox user. I started using Firefox for the added "security" and tab browsing. IE 7 has tabs, but it seems that EVERY "patch Tuesday" a security hole is plugged in IE by Microsoft.