Bier Circus in Brussels with a handy little photo attached. It's the same one you see to the right: what's on tap... an actual shot of the chalkboard inside the restaurant. (If you want to get on the Bier Circus email list too, just drop them a line here.)
Of course, lots of places publish their tap lists online. Most seem to be "example" lists of what they might have on at any given time. Just to give you an idea. It's better than nothing, but I like to see more effort.
Many places these days will announce special taps via Twitter, which is OK, but not as useful as seeing a whole list. And by "useful," I mean "mighty fun to look at while killing time at work and thinking about what you'll drink when you go out to meet your friends." Also, there is the ongoing problem that everyone on Twitter thinks that everyone is on Twitter.
Some savvier pubs frequently update their online tap lists, often by uploading the menu in PDF format. Examples include the Bridge in St. Louis (updated two weeks ago) and the Churchkey in DC (updated yesterday). Max's Taphouse in Baltimore has a "now serving" list on its front page, but I can't say how live it truly is. I'd guess that most of the prolific taphouses (read: bars with insane numbers of taps) have something like this nowadays.
What I'd really like to see is real-time stuff... I have vague memories of seeing live webcam shots of chalkboard menus... It might have been the Pelican in Pacific City, Ore., or the International Tap House in Chesterfield, Mo. Neither of those sites have them now, but I'm sure there are more. Maybe this is more trouble for the publican than it's worth. But if you can think of websites where you can see draft lists in real time, please post them in the comments below.