I've been collecting examples of paid online content, and have found five core categories:
1. Pay to gain
2. Pay to own
3. Pay for expert content
4. Pay for data/intelligence
5. Pay for convenience
More details available at: http://www.penmaen-media.co.uk/index.php/2010/02/what-online-content-will-people-pay-for/

I'd be interested in others' views and experiences..

I'm taking part in a panel at Publishing Expo on 25 Feb at Olympia on this topic - do drop by to listen in.  Should be a good session!

Carolyn Morgan