There are so many programming resources available. With that said, is there a single one that had the biggest effect on your code?
For me, it would be Practical Refactoring by Llewellyn Falco and Woody Zuill.
On the face of it, this video should be extremely dry -- it's literally just a couple of guys refactoring code for two hours. In spite of that, those two hours shook me to my legacy-coding core. Refactoring has since become vital to my work and (dare I say it) fun!
So how about you? Be it book, blog, video, or whatever -- which single resource has most affected how you code?