Internet exposure being a key element of my marketing plan and I make sure to discuss how I'm going to utilize that during my listing presentations. I believe in focusing on those sites that consumers go to so it's important to stay current on what those sites are. Here's the latest rankings of real estate websites (based on the number of visits) as published by Hitwise.
All are not be appropriate for residential sales since the focus of some of these sites is rentals and one (LoopNet) is primarily known for commercial real estate.
With syndication, posting a listing on a few sites can multiply to a hundred or more. Syndication brings with it a number of issues that have been thoroughly discussed by Bart Wilson here on ActiveRain.