[bluedevil2k]

Handy tip for software testing - Gmail ignores everything after a “+” in the address. So when you’re testing different accounts in your software you can use <youremail>[e-mail], <youremail>[e-mail], <youremail>[e-mail] etc. to create many different accounts in the software that all go to the same email address.

[cardamomo]

That's not just a Gmail feature. It's part of RFC 5233. See "Subaddressing."

[Terretta]

Indeed, also works at outlook.com / hotmail and others.