The folks at App.net were kind enough to give me a link good for 200 400 700 free accounts. If you want to join this awesome service, you can use the link above to sign up for free (while supplies last).
Update: The free invite codes are now used up.
And once you start looking for ADN clients, I use just one: Riposte on the iPhone. I hear Felix is also fantastic. And on the Mac, Kiwi is all the rage with the cool kids.