Top companies that offer online lead generation and management systems for realtors?

[Philip Accardo]

Does anyone have any experience with using any online lead generation services offered by various marketing companies. I am talking about the sites that offer potential customers the ability to search the MLS system as you automatically gather the contact information. Some example of the companies I am talking about is http://boomtownroi.com/
I know an agent in my area that users this service and he is pretty successful. I am looking for potential competing companies.
thanks!