ProfitTrain (formally Billable) was a time tracking and invoice tool for Mac OS X. While simple, it found an audience and was a very successful product for me.
Sadly when I joined the Shindig team my time was full of Shindig responsibilities and I could not find time to keep ProfitTrain up-to-date. I decided to sell ProfitTrain to RazorAnt hoping it would find some updates. It saw some, but sadly not nearly as many as I wanted. It was discontinued by RazorAnt in 2018.
Related Tech: AppKit, Mac OS X, Cocoa, CoreData