Back in 2006 when Zillow.com announced its plans to add for-sale listings, Seattle's largest real estate agency, Windermere, didn't want anything to do with it. The real estate company even expressed suspicion over the online real estate upstart's intentions.

"Anytime you have a brand like Windermere, you have to be really careful who you put your name next to," Michael Rahmn, an executive at Windermere told me at the time.

That's part of why a partnership announced today is so significant. Windermere -- with more than 30,000 for-sale listings in the western U.S. -- has decided to feed its listings directly to Zillow.com. The deal speaks to the growing power of Zillow in the online real estate business, which attracted 8.8 million unique visitors in August.

Real estate companies -- even those like Windermere who were at one time reluctant to play ball with Zillow -- are finding that they need to be on the site in order to help sell homes.

"We constantly look for ways to maximize marketing exposure for our associates and listings," said Jan Edmondson, Windermere Chief Marketing Officer, in a statement. "The Zillow Listings Feed program offers a tremendous marketing opportunity for our agents and their sellers."

Windermere last year signed a deal to provide its listings to Trulia, the San Francisco competitor to Zillow. So, because of that, today's deal is even more sweet.

As part of the partnership, the Windermere listings also will appear in Zillow's popular new iPhone application.

Zillow still faces an issue that the Windermere exec pointed out in late 2006. Unlike Redfin, Windermere, John L. Scott and other brokerage Web sites, Zillow doesn't have a comprehensive listing of all homes for sale. That's because -- in most areas -- it doesn't pull listings from the local Multiple Listing Services.

However, as Zillow inks more deals with big real estate companies like Windermere, that database will only grow. (It now has more than 3.7 million listings throughout the country).

And, perhaps one day, it will turn itself into a more user-friendly (and public) version of the Multiple Listing Services.