It's called a library but the gardens are the main thing- absolutely amazing and spread out over 120 acres.
Admission is normally around $20, but once a month there is a free day (as in free beer, no parking charge.) Trick is you have to order the free tickets online, and they go REALLY quickly. On the first of the month you can order at 9 AM, and literally by 9:08 that month's allotment is gone. The tickets are for the following month on the first Thursday I believe-
They have a bunch of different 'themed' gardens, the Japanese one is especially beautiful IMO-
The Huntington Library