I found an article that gives some pretty good techniques to avoid/prevent SPAM, Junk Mail, and Telemarketing. I'm pretty sure most of them are legal, at least right now :)
There's a website that will provide you with a disposable email address, it allows you to use it just long enough to get certain information from websites that you would prefer not have your real email address. Another one adds extra encryption to the mail that you send to prevent it from being intercepted/read.
It's not too long, so give it a look.
Any useful things you've found to keep your presence anonymous on the web?