Many state parks in California has a special area for cyclists and hikers at a special rate ($5 - $10) and no reservation is needed. 1 night only in each park.

AFAIK, primitive camping for a night or two in a National Forest is free. Just be far enough away from roads and water sources. And many national parks have national forests near them.

Also check out freecampsites.net

I did camp on private property a few times without permission and only got discovered once: It was at a church and the pastor gave me free breakfast.