Zillow, which just nine months ago added rental listings to its real estate search site has now added another 90,000 managed-rental listings to its database through a collaboration with Apartments.com, a Classified Ventures site.

The new listings include unit details, photos, floor plans and contact information for the property managers.

With two million of Zillow’s 12 million monthly visitors now apartment dwellers this is a smart move for both companies. Now Zillow ups its rental listing numbers to 150,000.

Zillow mobile will provide access to all the listings as well. So far two million consumers have downloaded the socially-enabled mobile app.

“Zillow offers an unmatched user experience for home shoppers who are renting, deciding whether to rent or buy, or shopping for a home to buy. This partnership deepens Zillow’s footprint in the rental search industry, and gives Zillow users access to a new category of managed apartment rental listings,” said Chloe Harford, Zillow’s VP of mortgages and new ventures, in the announcement.

With plans to go public in 2011, this is but the latest in a series of ramp-ups and collaborations that include introduction of IPad capability, collaboration with RedFin for real estate loan quotes, and a team-up with Yahoo Real Estate to take on the industry giant, Realtor.com

“Our new partnership with Zillow is extending our reach to renters,” said Kevin Doyle, SVP and GM of Apartments.com, in the announcement.  “By creating easy and convenient access to our listings, Apartments.com is able to drive highly qualified apartment shoppers to the leasing offices of our advertisers through unmatched exposure.”