As I mentioned previously, Google has open Gmail for free registration. No invitation needed. Here is the Gmail signup screenshot.
[ Thanks, Alan ]
Note, the free Gmail signup is for U.S residents only. If you are from other countries, Gmail invitation is still needed to open a Gmail account. You can get a Gmail invitation from me, visit Free Gmail Invitation page.
Update: Now everyone can sign up for Gmail without an invitation. New Gmail account has been open to ALL people in the World! Visit http://gmail.com/ for Google mail sign up.
Bonus: Amusing attached email signature
Usually the free webmail provider will attach advertisment signature to user’s outgoing email. Here is the email I received to request a Gmail account invitation (currently offer 2,500MB+ space).