The California Official Reports page allows you to search for California cases (for free) in the LexisNexis database.
You can do a keyword search in this database. The default keyword search method is the Natural Language search, in which you merely enter your search terms (without connectors, commands, or special symbols) and LexisNexis retrieves the most "relevant" cases (based largely on the frequency with which your terms appear in the cases), and ranks them in order of relevance.