We got this great idea from Ricky Martin from Issue #23 of Trafficology:

Many times when you're setting up a marketing campaign you need to use an email address other than your primary account. You can set up a new Hotmail, Gmail or Yahoo account but that takes time. Instead, Ricky suggests you try an excellent temporary mailbox at http://www.temporaryinbox.com. He suggests that you use this when you want to anonymously complete an online registration form as well as when you are doing marketing testing.
If you want tons of great Search Engine Optimization ideas, check out www.trafficology.com . They have tons of cutting edge techniques for quick growth!

Do you have any other free email account ideas?