The most useful development tool I use makes me more productive on lots of tasks all day long.  I can open just about anything I am working on, and get the job done fast.  It applies colors sparingly to my work, and helps me identify mistakes.  It helps me multitask, working on many things at once.  It understands dozens of different languages with ease, all at the same time, and you can add more very easily.  It has all the qualities of a developer who embraces open source solutions – it is fast, cheap easy to use, and available for download any time on SourceForge.  I have tried its competitors, but this one stands out among the crowd.  It was recommended to me by a colleague, and switching over was very easy.  My most useful programming development tool ever is Notepad++.  I have used the standard Microsoft Notepad, and I have used TypePad, but Notepad++ is head and shoulders above the rest.  Give it a try.  You will convert too. 

Disagree?  What is your most useful programming development tool ever? Leave a comment and be heard.