Block web sites on a timer

I’ve realized that I kill a lot of time by repeatedly checking web sites I just visited (SDMB, Facebook, Drudge) when there’s no reasonable expectation that anything would have changed in the past ten minutes. I seem to do so out of a combination of laziness and my fingers’ muscle memory.

I’d still like to check these web sites, say, once an hour. Is there any kind of app that would block a particular web site for a set period of time, say, for an hour after I last visited.

RescueTime does something like this, though only in their paid version by the look of it.