Open houses are transient things and nobody has found a way to corner the market for that information. As a result it is widely scattered over many web sites and telephone poles.

Craigslist.org has been a good source if you don’t mind the missing information. Search the RE ads for “open house”. If you can get an address or MLS number, go to the Redfin search page and paste that in- voila you have the listing and ‘generally’ reliable information.

Recently I’ve noticed improvement in:
http://www.openhouse.com/
– more listings are appearing there lately and the information is more complete (I still jump to Redfin for complete info). You can save your search criteria and they will email you a list of open houses in your selected area.

As a result of your question I looked and found this:
http://www.openhousessandiego.com/
– few listings at the moment, but maybe that will improve.

You can find a few listings at the San Diego Reader:
http://sandiego.backpage.com/HomesForSale/classifieds/
(inside house/condo) Search the ads for “open house”.

As far as ‘reliable’- I take only the day of the open house from these sources, everything else I report on comes from the MLS listing as found at Redfin.

best wishes
Tom