A friend of mine does it the hardcore way, through DNS. He adds MX records for subdomains corresponding to the name of the site he's signing up for. Lets say his domain name is example.com:
bob@hilton.example.com bob@ycombinator.example.com bob@firefox.example.com and so on
He also does it for friends(!) meaning when he gives you "his" email, it's always of the form bob@<yourname>.example.com
If he tires of you or your comms get too spammy, he just bins the relevant DNS record. No more mail from you!
