From: Danyel Fisher <danyelf@cs.berkeley.edu>

But I wouldn't want to get spammed myself, and I don't think anyone else does, either. So I'm looking for other good sources of subject pools. Where do you find sets of internet users?

I don't know if this is at all the kind of sample you are looking for but Survey Sampling, Inc. has Internet samples. While I myself have not used them I understand that they are recruited for willingness to participate in email surveys. I use Survey Sampling all the time for Random Digit Dialing and listed sample telephone surveys and have been quite pleased with their products.