For those that have gone into the field of software development, what would you say is one of the most interesting programs you've created?

I've been lucky enough to have worked on several interesting applications.  I've written an application that worked with "Video for Windows" doing some blue-screening, I've worked in the 3D realm (combination of C++ & VRML), I've worked on several hand held devices, and I've built views that deal with maps (Geo Coordinates - that sort of thing).  I'd say the mapping was the most interesting & fun.

The most interesting programming I've done, was working on a team to develop flight software for an airport. It was the most challenging, but also the most rewarding, when I consider the positive impact it had for society. The program that I had, and continue to have the most fun with, is a artificial intelligence program for a robot that I built.....

I think the vast majority of professional software development is collaborative, so it's pretty rare that you create something from scratch. Even in the open-source world, it's far more common in my experience that you modify an existing thing because it doesn't quite do what you want it to do; I have a whole page listing various hacks I worked on 20+ years ago, mostly in C (because that's what most existing software was written in back then), but also a few in x86 assembly language, IBM's REXX, and various Unix shell scripts.

Some of the most fun code I've written lately is JavaScript using three.js to make 3D models of stuff. I have a lengthy post describing and linking to an astronomical model here, and more recently I built a pair of very simple models of a ski trip based on captured GPS data. But those are just visualizations; people have written entire games in three.js.

Professionally I mostly do back-end work ("plumbing") on distributed data systems these days. It's mostly interesting because of the scale (thousands of machines in multiple data centers, with multiple server or app instances running on each host, and multiple DBs and partitions hosted or consumed by each of those instances). The recent code I'm most proud of makes debugging problems across all of those data sources and partitions a lot easier, but it's the sort of thing that doesn't sound too exciting unless you've felt the pain of not having it in such a situation. ;-) But back in my corporate research days, I spent a few years doing virtual worlds research, which was very fun and directly led to my personal interest in VRML and later three.js.