Jetable: Disposable email, for a single usage
Jetable gives you a relay address that work for 1-6 days. A French web site, it's free for now. Useful for many purposes, but since it expires it's not a great choice for site that wants an email address as a username (NewEgg, for example).
Mailinator needn't expire, but it's public, albeit hard to find. Not a great place to store data you want to keep private (credit card information, etc).
I actually do ok using a Yahoo account for this type of thing, but I think I'll try jetable.
Spamotomy lists a number of similar sites. This is an amazingly active anti-spam site. They list a lot of anti-spam tools, but I think they're emphasizing a failed approach.