I've got many — perhaps too many — hobbies, which often involve building, designing, or otherwise working on things that take a long time and might be classified as projects.

Software

Reverse Density Altitude Calculator — Stemming from a discussion a few years ago regarding trying to fly to the top of Class E (17,999ft MSL) in a Cessna 152 on a particularly cold day (which we later realized may not be the best way to do it) and my experience writing weather ETL for Understory, this "reverse density altitude calculator" calculates the height where density altitude equals a specified altitude based on temperature and geopotential height values from the NCEP forecast models. Source on Github.

Non-Software

Flight Simulator — A few years ago I set out on a project to build a flight simulator so I wouldn't have to rely on virtual instruments and keyboard controls. The idea was simple: plenty of old airplane instruments and controls are available on Ebay for not much money, and filling them with servos or other motors and potentiometers and interfacing them with a computer shouldn't be too hard. All that's left after that is to calibrate the readings and interface all that to a flight simulator software. Shouldn't be too hard, right? Alas, I think the greatest reason it's still not working is my lack of effort. That's starting to change though, as I recently started a CAD drawing for my instrument panel and will be designing the rest of the hardware in the coming weeks. There is certainly a software component but it is not far along yet, and does not make up the majority of the project — I've got starting code for the airspeed indicator on GitHub, though most of that is currently a fork of a Switec X25 Arduino library.