[ben1040]

SA is pretty brilliant in how they charge $5 for changing your avatar image, something which 99% of other popular forums let you do for free. For $10, you can change another user's title. If you get in a flame war with a guy with ten bucks to spare, he can buy you an embarrassing title/image. You have to either live with that or cough up the $5 to change it back.

[HNer]

This idea is actually very clever. This is the basic concept to make money from free, 1st you solve a problem and generate plenty of traffic, 2nd you indirectly or directly create a secondary problem for your users. This is the part you monitise. According to the variables and the ticks you use to create the secondary pain, will adjust the revenues you make.