[gradschool]

The most ingenious business model I ever heard of was for a proxy service called hide-my-ass that let people get past firewalls when web surfing at work. They didn't charge a penny for using it, but if a user visited Amazon, the proxy invisibly changed every link on Amazon to one of the company's own affiliate links.

An unusual business model I've always liked but never got around to trying would be to have a stock trading game on iOS or Android where people bet on the directions of hypothetical charts (that are secretly derived from historical data) and compete for the best returns. Give the game away for free with no advertising, mine the results for outsized performers, and if you find any, then secretly feed current market data to them and trade off their bets. I'll bet there are lots of other possibilities for getting useful work done for free by gamifying it.