x.minutes.at: make a time-limited visit to any website
So, if you wanted to read Download Squad for 20 minutes -- hey, feel free to stick around longer, this is just hypothetical -- you could go to 20.minutes.at/downloadsquad.com, and your timer would automatically be set. Follow a similar formula of the time limit (.minutes.at/, and then the site address) to set your timer for practically any site on the Web. It doesn't work well with Gmail and Google Reader, unfortunately, but most anything else is fair game.
x.minutes.at also offers bookmarklets to time-limit the current site, and you can use it in stopwatch mode by entering http://howmany.minutes.at, instead of a number. I'm almost afraid to use stopwatch mode, because I don't want how much time I spend at TVtropes.org on a given day.