SPICEBAG A SPICE-like circuit simulation program written in Python and FORTRAN with a pretty decent GUI. We did this as a university project and it involved some serious (for us at the time) numerical algorithms and software engineering.
Domotique A home-automation system simulator in C++ and Gnuplot we wrote in EPFL. It has what I believe to be an excellent class model and a perfect use of templates.
Chuckles An absolute mess of a thing in C and x86 assembler. The merest baby of an operating system, but it's been a lot of fun structuring and writing it and exploring all the freedom a kernel developer can have.