Hays, Montash, People Source Consulting, GCS Recruitment Specialists, Glocomms, Third Republic, Citrus Global, Gulp, iPAXX, Modis Contracting Solutions GmbH, Etengo, Templeton Recruitment, and on and on and on... :)
Hays is a huge one and I've done one contract through them which went very well. They seem to be well organized, know when I'm becoming available and more or less what kind of work I'm looking for.
Computer Futures on the other hand spam me with tons of irrelevant stuff. That is partially my fault however, since a long time ago I submitted a CV to them with lots of keywords that I don't care about any more. Having said that, I recently got an interview through them and the client wanted to hire me. Ultimately it didn't work out, but I don't think they are to blame for that. I certainly did not put them on the blacklist. :)
Some of the other ones are smaller agencies from the UK and Ireland, where I am regularly in touch with one or more of their recruiters. You can probably find all of them on LinkedIn. The good recruiters often post new contract opportunities including rates (a nice habit in the UK, still uncommon in Germany).
They all love phone calls, which is a bit annoying, but not the end of the world, IMHO.