I do a fair bit of computer programming. Computer experiments are often pretty useful in my research.

Probably the biggest project I've carried out is not mathematical, however. I did most of the C/C++ coding for the WinDraft document assembly system at Eidelman Associates. WinDraft was a system that made it easy for an expert to build a series of questions, which could be used to automatically create legal (or other) documents. It's kind of like US-based tax software, but more focused on text.
For a while, systems based on WinDraft were pretty competitive in the estate-planning market (creating complex wills and related documents). You can see a review from back in the day here; and a comparison of WinDraft with some competitors is here.

Here are some other things I've created.

Mathematical code

Other software related to mathematics

Older stuff

These are both from Windows 95 days:

And this one is even older: