Lastminute.com (the UK version) has opaque Top Secret hotels. Except that the descriptions they give are usually so good that you can figure out which hotel it is.

I just read that Londontown.com now has Top Secret hotels.

Hotel.de sometimes has a feature (I think it has Roulette in the name) where you will end up "winning" one of 9 Mercure hotels around Berlin, but you don't know which one. This comes up when you do a search for Berlin. I haven't seen it for other cities. I saw a variation of this roulette idea on the SkyEurope web site some time ago, where you could end up with one of three Prague Ibis hotels. These are one-offs, so you just have to keep watching.

EasyclickTravel has opaque hotels.

Eurostar.com sometimes has opaque deals combined with Eurostar tickets to Paris (and possibly London or Brussels).

Wotif.com apparently has some opaque options.