Last year I randomly decided to improve my maths skills, and so I turned to Khan Academy to show me the error of my problem solving ways. It didn't go too badly, once if figured out how to use the scribble space.
I'm still doing it, but you can now plough through the skills in grade order, so I am currently ploughing through 4th Grade maths. Apparently I have 97 skills that need improvement. I feel substandard.