[chrxr]

For those like me, and like many in the UK, whose maths education ended at 16, I can definitely recommend Khan Academy for learning or relearning pre-calc stuff. The videos are short, the exercises are well thought out and numerous, and importantly the UI is great.

For example: https://www.khanacademy.org/math/algebra/

[alexgmcm]

It's good for calculus too - I remember watching his videos on calculus before I went to uni 10 years ago so they may have improved even more now.

[chrxr]

I remember looking at the site a few (maybe five?) years ago and everything was a bit of a mess. They've really pulled it together since then. It's really great.