Great website for home searching, recent sales data, and foreclosures; very robust (except it can't filter on schools)
Find All the Homes for Sale & Experienced Real Estate Agents | Redfin
Southern California Home Resale Activity
Case Shiller home price index
Cool zip code map here:
The Most Accurate Free USPS Zip Code Map Anywhere!
City-Data's data display for a zip code, very good for finding recent sales
Another forum similar to City-Data Forum:
West Coast and Interior West - SkyscraperCity
School performance maps:
California School Performance Maps
Want to compare weather between two different cities?
City Climate Comparisons
Powerful Census data maps (though the data are old):
Tiger Map Server Browser
This tool mashes Craigslist housing listings with Google Maps. Very neat.
Capistrano School District Elementary, Middle and High School maps (in hi-res PDF and JPG formats):
Attendance Boundary Maps
Saddleback School Maps (not too useful, no boundaries):
Saddleback Valley Unified School District
Several OC Google Maps with some gated communities, school zones, other random layers:
National Honor Society
Some Google Earth KMLs with demographic and census data (haven't checked them out; if they suck, I'll ask the mods to delete this)