The simple tool I'm most proud of is spiped. If you have two systems which need to be able to talk over TCP without worrying about evilness on the network between them, spiped is the answer. The simple tool which has probably had the largest impact is bsdiff -- now found used in hundreds of millions of devices -- but I'm not particularly proud of it because it was a quick hack and horrible code written by a C novice.…
Ludicrously simple but I rely on it and use it every single day for Dev (and real) purposes
edit the app I use for HN (MiniHack) seems to have mistakenly replied to the first reply instead of the root
Sorry for comment sniping!