The basics of good UI design and communication (aka graphic) design.

I see so many apps and projects ruined by bad design or poor usability. Just learning the basics can make a world of difference. Plus the visual problem solving techniques (i.e., how to communicate a concept visually) are a stimulating challenge that should open your eyes to new ways of seeing, which should in turn have an impact on your code.

A recommended book is The Non-Designer's Design Book by Robin Williams

Here's what Joel Spolsky says of it:

Wow! Everybody has to do some graphic design, and not every software team has the luxury of professional designers. This excellent, thin book will give you a grasp of the principles behind page layout, fonts, etc. The good news is, you can read it in the bath before the water gets cold, and the next day, your dialog boxes and powerpoints and web pages will start looking better.