Math
I didn't study any maths during my University degree. Some statistics, but no maths. School turned me off it. I wish I'd studied it at least a little, because then I'd at least have had some idea of how much I don't know, and (hopefully) where it's most applicable. So, now I've taken it upon myself to learn some of it, as it relates to my field of interest - computing and programming, in the hope that it will expand my horizons and make me a better programmer.. and generally keep me busy and productive.
So, I did a little bit of research. It looks like Concrete Mathematics might be a good book to buy. A little pricey, but probably worth it. It is largely a slower introduction to the Fundamentals part of The Art of Programming by Donald Knuth. The Art of Programming requires a lot of effort to follow even the basics (my current method is to implement each algorithm with Python, which I then grok), so I hope that Concrete Mathematics will be a lot gentler on my poor math-deprived mind.