I don't normally suggest ways to “cheat” a system, but some people seem to really want some more extra GMail invites to share with stuff that matters to them. So here's the deal: You finally got invited to GMail. You invited two friends already, you have one invite left. You want to share it with a good cause but one is just not enough. What do you do?
You invite yourself. You then create a new GMail account from that invitation. One day later you get 3 GMail invites available to you to share with good causes. Want some more ? invite yourself a few times more. Multiply the number of new accounts by three and see where that gets you. Obviously this should come with a warning: don't abuse the system for bad causes. But heck, this was so easy to figure out I just had to share, especially for those seeking out to do good. Until Google finishes the popularity contest or figure out this loophole - I guess it's ok to use it.
Another GMail tip: You can't send in ZIP files. na-ah. However - you can send in RAR files.