Take a programmer that has never used source control, show them what it does, and their eyes light up... the benefits are obvious but until they actually see it most people had never considered the existence of such a tool.
What other such things exist? Tools or approaches or techniques that aren't obvious before you encounter them, but once seen have obvious value. Things that are likely so ingrained in the way you work that it's hard to think of working without them, things almost embarrassing to bring up because you expect the other person to say "duh, that's obvious!"
No matter how petty something seems there is a chance that other people don't know about it yet; I'd like to get an idea on what things I'm missing simply because I never thought of them.