• GMail
• Yahoo! Mail Plus
• Rackspace claims to support, but not verified

Subaddressing for email (also called "plus-addressing") is the practice of allowing an email account extension using the plus character, giving you an unlimited number of unique email addresses which are all part of the same account.

Given an email address joe@example.com. Joe can add a plus and any string of characters to his username (the thing in front of the @-sign). Mail sent to the address joe+pizzahut@example.com would be delivered to Joe as well.

This allows Joe to sign up for mailing lists, without the danger of being attacked by a large amount of spam. If lots of spam appears coming to this mail address, it is easy to set up a filter to pipe that mail to the trash. As long as no spam appears, that address functions identically to his regular address.

References

• IETF RFC 5233 on Subaddressing
• Devaluing Email Addresses
• Dealing with Spam