These are projects that are quite old but for completeness and perhaps for nostalic reasons I added a few notes about them here as well.
oCADis
oCADis was my first major application I did. It is a 2D CAD application heavily inspired by AutoCAD. The original version run on Linux using GTK as framework. Actually, I think the very first version I made was for Atari Falcon030 but never got to release it before I switched to Linux. I spend many hours on the software and got to a point where it was kind of useful. It had the basic drawing tools like lines, circles and even dimensions. After I published it online I got a few people to send in bug reports and provided me with code patches to expand the application with new features.
The GUI of oCADis was inspired a lot from GIMP but I was never really happy with that. So I started with oCADis 2, a total rewrite of the code with a new interface. Still on Linux and still using GTK as framework.
I never got to release it but in general working on oCADis was a great experience and I gain a lot of knowledge about graphical mathematical algorithms, e.g. zooming, finding a point on a 2D line or calculate a circle through 3 points etc.
Syllable
Syllabe was a free open-source operating system and was forked from AtheOS. I found Syllable when looking for alternatives to Linux and was caught by the speed and simplicity that the OS had to offer. I started working on creating desktop applications and later I got more involved in the internals of the OS.
Dock in the bottom 
An extended filebrowser 
Updated text editor 
oCADis running on Syllable