A free option might not even be necessary, just a "See how it works" page would probably suffice. Take us through a common use case, maybe even use that convention scenario I saw in Saaspire's blog. Take a picture at a convention, upload it to ThingCounter and ask us to count heads, we get back to you with a count of heads.

A video would be even better. Videos are very easy to consume, and it is typically very easy to win people over with a well-done video.