Google Mail changed their interface for invitations some time ago, and now its apparent why: Most, if not every, Google Mail user now has 50 invites to give away!
The question now is if this is a prelude to opening Google Mail up for anyone to register. It might be that Google will continue only allowing new users by invite, there are some obvious advantages to being able to control just how many new users enter the system, and the trail back to who invited them. The trail could possibly make it easier for Google to close spammer accounts.
Oh, and while I remember: If you need an invite, leave your name and e-mail address in the comments section.