On Sneakemail... good solution, as long as people keep in mind that the disposable email provider needs to be at least as reliable as a regular email provider—if they go down, you might get locked out of your accounts; if they get bought out (or hacked), the new owner (or "owner") has the ability to log into all of your accounts, and access to any sensitive info that has been emailed to you, besides.

My preference is to have my own domain name and configure a catch-all for email, which allows me to make up addresses on the fly. But a lot of services, including GMail, also allow you to use plus-addressing to similar effect.

(Of course, even if I register with different emails, an attacker can still see that I registered with emails on the same *domain*, so if I really wanted to make sure my identities weren't linkable I'd use a different domain or even email provider to register the other account.)