Does exactly what you'd want (unlike a couple of alternatives I tried.) Takes a few minutes to set it up how you want it, but is flexible enough to either block specific sites, or block everything except for listed exceptions. It can block them through your workday, or after you've been on them for X minutes/hours, or a combination of the two. You can hit a button to override the block, or you can configure it so that overriding requires a password, or is not possible at all!

For those who use profiles: I use one Firefox profile for work, and one for personal stuff. I installed LeechBlock on the personal one, to block everything during my workday. This seems to work well for me.