I think MTB Bill's is hands down the best site I've run across. If only we could clone him, and station a clone in each region... I find a lot of bogus information on some of the user input trail sites (not the gps-based ones), and don't want to pay for trails.com or anything like it. I get my best information from google and regional forum searches. I want details, not just a star rating. For forums in the US, MTBR is good, and for SoCal, well, you're on it...
VC Trails An absolute winner of a site is www.venturacountytrails.org. It has trails in Ventura, Los Angeles, and Santa Barbara counties. The details for each trail or trail system are extensive, and the 3D topos are excellent. When I've explored new areas using the resources from the site, I always feel super prepared.
I've done a lot of route mapping for geoladders outside of SoCal and rely on a huge collection of guidebooks from all over the world that I use when I'm out on the road. I also use the trail reviews on: http://www.dirtworld.com/ and http://www.mtbr.com/ Just to get an idea of what the locals think of different areas. Some other sites I've found: Colorado: http://netcairns.com/wiki/index.php?title=NetCairns Minnesota: http://www.morcmtb.org/ Texas: http://www.mountainbiketx.com/ State Visitor Centers are a good source of information and maps. I also usually stop by local bike shops and ask their opinions of different trails.
Added a review for Bonnelli Park (Puddingstone) on whereweride.com You can see it here: http://www.whereweride.com/mountain-bike/california/los-angeles/bonelli-park/Default.aspx?action=1 -CL