Open Source C Business Software

GnuCash is a personal and small-business finance manager with a check-book like register GUI to enter and track bank accounts, stocks, income and expenses. GnuCash is designed to be simple and easy to use but still based on formal accounting principles.

PNotes is light-weight, flexible, skinnable manager of virtual notes on your desktop. It supports multiple languages, individual note's settings, transparency and scheduling. Absolutely portable as well - no traces in registry. PNotes.NET edition requires .NET framework 4 Client Profile

Viking is a free/open source program to manage GPS data. You can import, plot and create tracks, routes and waypoints, show OSM, Bing Aerial and other maps, geotag images, create routes using OSRM, see real-time GPS position (not in Windows), make maps using Mapnik (not in Windows), control items, etc. It is written in mostly in C (with some C++) & the GTK+3 toolkit. Viking is currently dual managed - with a corresponding Github presence at: https://github.com/viking-gps/viking Where you may find it easier to view the code and raise issues there too.