Check out this article if you want more power and flexibility out of your gmail account.
It shows you how you can use one email account with different sites by using a . (period) or + in the email address and you can generate a number of email address combinations that can be used for different needs (such as for bills, friends, social networking sites, registrations, etc.). You can then track who is selling your email addresses!
Mailinator for disposable email addresses
Another option I like for disposable email addresses is Mailinator. It allows you to get a dispoable email address instantly (without any registration). Check out their site. Just keep in mind that the email addresses are really disposable and emails are deleted frequently so don't use it for anything longterm. It is best used for sites that require registration to post in forums where you don't care about the privacy of emails from the sender.
As an example, if I want to use an email address like email@example.com, all I have to do is just use this email address whereever I want (without any registration) and read my Inbox by going to http://www.mailinator.com/ and keying in my Inbox address "mobilebookmarksblogspot". It's that easy! Note that all emails sent to this address will be deleted within a few hours and that anyone (including you out there) can read this Inbox contents.