Ask HN: What are the best resources you have used to teach yourself mathematics?

I feel that my background in math isn't as strong and well-rounded as it should be, so I'm requesting some assistance from all of you. If you've used any sites, books, or listened to any important recorded lectures that you wish to share, please do so.

The obvious resources are Khan Academy and such, but I'm looking for things that might not be as distinct or popular.

Thanks!