Bit of a LifeHacker type post here, but here is a new trick that I have found useful for a lot of things. Many times I need to use a newly unique email address for something but do not want to go through the whole signup and captcha hell. Maybe I am testing code or more commonly need another email account for a service to signup for another account. With Gmail you can just append "+something" to the end of your existing email address and it will still go into the main inbox. You can even create a Filter on it to sort it better.
All three of these emails will go into the same inbox.
email@example.com firstname.lastname@example.org email@example.com
I use this to create duplicate Twitter/Facebook accounts for testing and also just recently to setup another Amazon EC2 account so that I could get another Free Pricing tier =)