Social Networking Sites are a must to promote yourself, your company, and products/services. I would also recommend article posts and pr sites. Here are a couple networking sites I've found useful.
Active Rain: Free Networking site for real estate professionals
Linkedin: Online community with 9 million+ business professionals
Ryze: 250,000+ members, special networks relevant to your specific industry
WannaNetwork: Network for Real Estate Professionals.
Delicious: Social Bookmarking Network
Technorati: Nework of National Blogs, 7 million+
The Real Estate Bloggers: National Real Estate Blog, spotlights news articles and video clips
Kazeli: Business Network
Squidoo: Create your own lens