One, which I have known about for a while, but of which I was reminded by early ThoughtStreams user, Joel Dueck, is the one-off charge that increments by number of users.
Pinboard has used this with much success.
Now, Pinboard charges $0.001 per user that preceded you and they're now up to $10.35 at the time I write this.
Some have expressed concerns about the linearity of this so one variant would be to go logarithmic.
Without any multiplicative constant, the natural log actually has some nice characteristics here. The tenth user would pay a one off $2.30, the hundredth $4.60, the thousandth $6.90 and so on.
Of course, this does raise the question of how people who have already paid more than this would feel. This site is what it is because of the great thoughts the early users have posted and I don't want to screw them in any way.