Greg's House of Treats

http://itcr.nci.nih.gov

The big ones are:

http://zillow.com/

http://trulia.com/

http://redfin.com

There are some interesting thoughts from real estate agents on these sites. 

http://www.inman.com/2014/03/27/what-to-do-when-seller-complains-but-zillow-says-my-house-is-worth-more/

N.b. the above article mentions three other AVM that you can use to compare with zestimate.

http://www.businessweek.com/articles/2013-03-07/why-redfin-zillow-and-trulia-havent-killed-off-real-estate-brokers