Nybbl is an interesting product of the recent RailsRumble. Although still in its infancy it looks like it could be an interesting way of sharing knowledge about specific topics - some free, some not.
Access is currently through your twitter account, but once in you can have your nybbls' delivered through your iPhone, SMS, or twitter. I wish it worked for me, but at the moment I just get errors. Hopefully that will be fixed soon though!