I like Allen Tate's as well, due to the many ways one can search for a home (even if you don't know the magic "zone" number).
Also, some sites will e-mail you when properties come on the market that match your criteria.
Of course, City Data's one of my favorites as well! I love how much detailed information you can get about homes, neighborhoods and schools in one place--good and bad. It's so nice to have honest opinons from those that have been there (or are there, lol).