I was about to ask this same question so instead I'll contribute to this one.
One of the things I use is a Firefox add-on called
. I use it to block access in Firefox to "screwoff" sites like Slashdot, Fark, Shacknews, etc.
Of course this doesn't preclude the idea of opening up another browser to open those sites but since I have to have JavaScript debugging turned on in IE and every site on earth trips the debugger for idiotic reasons (Line 1: Invalid character, etc.) and using Opera is just an exercise in pain, it keeps me from going to the sites much.
What I'd really like to have is a program which blocks certain sites, globally, except for certain times of the day. Like, browsing over lunch only or something. If anyone knows of one like that, I'd love to hear of it.