I'm doing some research for an article and would like your help:

What startups and niches use payments or subscriptions as their primary revenue model?

The immediate one that comes to mind is groupon, but there are plenty of others. Like dating sites for example. Even 'service' sites like pandora.com fall under this.

What startups, niches or ideas can you think of that follow this revenue model? Please list as many as you can.

P.S. Lets leave out those that sell and ship physical products.