Redfin, whose UI I liked more than Zillows back when I was buying a house.

There's also realtor.com, which is the official site of the National Association of Realtors.

There's StreetEasy here in NYC... oh wait, that is ALSO owned by Zillow.

There's also openlistings. I don't like their search function as much as Zillow's but you can still find a decent selection in some markets.