All of that said, and although I know this is not the conversation you're looking to spur, I think I will stand by my previous vote for the most valuable programming tool (or technique) being the comment. Complete comments, in my opinion, are invaluable, not just to others who come behind you but to the original author too (six months later).

that's a given, but, not to nitpick, I'd call commenting a practice, rather than a tool.

the most often heard argument against commenting I hear is "I know what it is doing"

I'd like to give everyone making that statement a piece of code they wrote 5 years go and say "add this"

Erik