[jmonegro]

If you're on gmail, you can place a dot on any part of the username, and it will redirect to your account. For example,

[e-mail] [e-mail] [e-mail]

etc. all go to the original [e-mail], but pretty much every website will interpret them as different emails.

[uggedal]

On gmail you can also use a plus sign after your username: [e-mail]

[a-priori]

This isn't a GMail feature. Most (but not all...) email servers allow this.