GPSBabel has been around since the relatively early days of open source and we use lots of tools by others to help make us awesome. We've grown to rely on many services that just weren't around when we started, so we really appreciate our relationships with the following.
Appveyor provides the continuous integration that we use to ensure our Windows builds always work and are ready to ship.
Jetbrains provides a vast array of tools, but we're thankful for CLion's tools for refactoring code and powerful analysis tools.
Macstadium provides us with a dedicated Mac in a hosted environment. Enterprise-class Mac hardware in a datacenter solves a lot of problems for us. (Like being able to afford Macs from free software...) This service absolutely helps us deliver GPSBabel on MacOS.
Travis provides us with GPSBabel's continuous integration. Their build system dutifully tests and runs every proposed change on at least seven combinations of compiler, operating system, compiler, and such. Every change is tested on combinations that take at least two hours of machine time, so having this done in their data center is super helpful to us.