For Twiddla, I make a point of giving free accounts to teachers and students (or basically anybody who bothers to write an email and ask for one). This leads t

For Twiddla, I make a point of giving free accounts to teachers and students (or basically anybody who bothers to write an email and ask for one). This leads to tons of word-of-mouth referrals, and I'll sometimes watch as an entire school comes on board, one class at a time, over the course of a few weeks. People tend to write about this (and they like the product), so it finds its way onto the radar of companies who do Online Tutoring. We have an API they can use to create and embed our whiteboards for use as online classrooms. That's the bit that costs money. It's a tiny fraction of our userbase, but it accounts for nearly all the revenue.