I have a site that gets heaps of crappy hotmail registrations.
I've been using mollom for a while and it works OK, but I still sometimes ge 20 useless registrations a day.
The site doesn't require email verification to make the ecommerce smoother.
After months of frustation, I discovered the Honeypot module, no crap registrations since install!
I wonder if real people can still register? Hmm, have to check that.