When I set up this site I set it up so that I could create a myriad of fake email addresses that I then used to sign up for services. This let me keep track of which companies sell email addresses to spammers, and I'm actually happy to report that for 3 years now, with the vast majority of companies, I've never ever had an issue, and I've signed up for a lot of services. However, this weekend, I got the first spam to one of these accounts from a "legitimate" company: BookSpan, or Book-of-the-Month Club, which runs sites like the HistoryBookClub.com and ConservativeBookClub.com, both of which I've belonged to in the last 3 years.
The email was for HistoryBookClub.com and I'm shocked and dismayed, but I'd like to state it clearly:
BookSpan sells your email address to spammers. HistoryBookClub.com sells your email address to spammers.
Pass it on.

Just a techie note: you can get the same job done (i.e. have a myriad of email addys all converging to the same address, but still letting you figure our who sent it) from these guys:
www.spamgourmet.com
for Bookspan, for instance your email address could be something like bookspan.5.maurice@spamgourmet.com
(the 5 indicates te max number of emails that will be forwarded .. the rest will be gobbled up. you can change this at a leter date too)
Hello,
You may be blaming the wrong folks. You'll find the most likely culprits to be hijacked mail relays. The hijackers are known as pharmers, similar to phishers. They sniff the mail stream to capture email addresses and write them to a file and wait awhile before retrieving the booty.
I use a very special setup for all my email, and found occasionally only email addresses I used to send to myself have gotten pharmed. This is the only way for it to have happened short of someone breaking into my home and sitting in front of my PC with a fistfull of passphrases. I'm as protected as a professional Windows guru with a penchant for security can be, so there are no virii, no spyware, no hacks, no rootkits, no trojans, and no key recorders, etc.
Unfortunately, there's nothing you and I can do to combat these threats except use disposable email addresses, bounced at the server level, which is what I do. If an email address gets compromised, I just filter it at the server level and change the email address at the specific organization to whom the address is assigned.
Cheers,
Robert Reese~