Here's a NYTimes article about RentHop: http://www.nytimes.com/2009/02/08/realestate/08rent.html
I'd say over 80% of what you see today was done after we left our jobs to do this full time.
The interactive free-broker-service angle that the RentHop guys are taking is perplexing to me, but I must say it helps them stand out. I hope the approach does not hinder them from scaling up to the point where they can offer a better range of listings.