Although App.net has had only paid account tiers thus far, we initially conceived of App.net as a freemium service. It took some time to get to this point, but we are now ready to make this vision a reality.
The details are pretty cool. Free accounts require an invitation from a paying member, can follow a maximum of 40 users, and get 500 MB of available file storage, with a 10 MB-per-file limit.
Speaking of invites, here are two: