I think the ones listed here do more contract/temp assignments...
My experience with headhunters is that very few have the inside track, and they will usually get in touch with you through friends/contacts when they have something they want you for. At that point the "registration" process gets completed and you may go in front of clients for interviews.
Any outfit that charges YOU to register, etc....beware. Most likely revolving door jobs, overseas assignments copied from the web boards etc.
As GF said - don't plan on hearing about many of the decent gigs unless you have quite a bit of time in type, Management experience etc. Clients pay a pretty large cut to these headhunters.
Good Luck!