Are you looking for an alternative to the likes of Alt or Bondage.com? Then check out CollarMe. It is completely free as far as I can tell, there are lots of people on it (though please be careful of scammers). It even looks very nice. I understand that the forums are pretty good too, though I haven’t used them.
If you have some experience with CollarMe, let us know in the comments section.
The House of Void 
That place is FULL of scammers, so WATCH OUT!!!!!!