Slightly more advanced trick is exploiting the fact that Gmail ignores . in addresses, so that [email protected], firstlast, fi.rst.la.st, firstlast.., are almost infinitely unique encodings of the same mailbox. But again, smart spammers can just remove all dots so that the spammee can't tell who sold the address.
Surprisingly I've only seen one misuse of email address doing this for a few years. And I sign-up to competition and the usual suspects quite regularly. The bigger one I find abused these days is phone numbers. I guess it's harder to track source hence more dodgy brothers activity here.
This evades sites that filter on +'s and spammers that strip them out to anonymize their list. Also if they strip out the dots to further anonymize it, they just create invalid email addresses.
Admittedly this only works because not everyone does it.
Then I setup an forwarding email server, which forward <anything here>@mydomain.com to my main email. So I would register at sites like [email protected]. Sometimes, if required to send emails from registered email then I would create a new email and send.
Now, I have switched to Abine Blur and free version works fine for me. They have a chrome extension and it generates random email whenever you are filling the email field in web forms. (or you can generate random emails at will). Only caveat is, you cannot reply from that email. So, in such cases, I login to the site, change my email to something else and compose emails.
I suggest everyone to using Blur or similar tools. The guys who are running your site will ever know your real email.