This is a list for people who like having fun outside and by that I mean, running, hiking, camping, climbing in those places - and preserving all this stuff so we can keep doing it and pass it on to our descendants. (And don't forget good food and drink.) This list includes some older resources that I still found immensely useful (plate tectonics aren't THAT fast.) They also include resources which aren't for California specifically but have a lot of good CA stuff and will also help you plan road trips and vacations!
Of course, here are my own best-of lists for San Diego and SoCal generally
For urban trails, beer, and shenanigans, the Hash House Harriers