Isn't that what App.net is supposed to be? Or are there problems with it I can't identify (being too poor to deem $5 a month on a social network a valid expense).
The main issue is that the cost barrier stops people from signing up just to try it out, in in that sense that app.net really ought to have a 'free' tier. Even if it was particularly restive, it would let their users get a feel for it before paying.