There is little that makes as happy as seeing people use your software. At a small local LAN party, I saw people casually using npush and npoll (from my ncp package; people whom I never saw before and who didn't even know me. That was amazing.
And it's even more amazing to get bug reports that show people tried to do more with your code than you did yourself! The main problem with free software is that it is not very rewarding. Most emails are gripes about bugs or licensing issues, it's rare to have someone write you just to tell you that he likes your software. So getting a bug report about a detail of a library routine that is not exposed by the surrounding project is a very special gift, because it shows someone not only downloaded the code, he actually read the source code!
Anyway, I finally got an account on an ia64 machine, which allowed me to diagnose the problems the Debian build system reported on that platform. It turned out to be a bug in the start code, so this opportunity forced me to read about ia64 assembly language. I wonder which planet the designers of this architecture came from, and whether it even was in our galaxy.
My friend Öc is currently solving the holy grail of qmail; he patched qmail to add RCPT TO batching, and he is now working on a generic filter infrastructure, which is a big problem for integrating spam or virus scanners currently.