http://www.Trulia.com , http://www.Zillow.com , http://www.Craigslist.com , http http://:www.GoogleBase.com
Agent-2-Agent Resources ?
Website Systems ?
Multi-User blogging websites, single property websites, agent websites and online flyers.
If you are looking for websites for realtors in terms of listings, Realtor.com is definitely a good choice.
In terms of information for real estate professionals, you should check out TopProducer.com. They have a lot of information about marketing and other resources for realtors who want to improve their business.
Hope this helps!