Oakland Museum White Elephant Sale
It's billed as "the biggest, the best, and certainly the most enjoyable rummage sale in Northern CA," and while I haven't attended too many other rummage sales, this doesn't sound far fetched. The (carefully vetted) pile of goods absolutely stuffs the 96,000 square foot warehouse, which is sorted into themed departments. It's insane, and an excellent place to find weird and wonderful items for your next project.
Pro Tip: If you go near the end of the day on Sunday, you'll find discounts on the remaining stuff - some departments will charge a flat fee for everything you can fit into a paper grocery bag.
Bonus: all proceeds go to help the Oakland Museum, which is currently undergoing renovation. I'm a huge fan of their natural sciences exhibit, which features some excellent dioramas showing California birds, reptiles, and mammals in their native environments. Truly good stuff.