There is a wide variety of current applications and libraries for different audiences. The following list calls out a select few officially supported projects. A list of community maintained projects can be found in the Mozilla wiki.
Mozilla Stumbler for Android
Mozilla Stumbler is an open-source wireless network scanner which collects GPS, cellular and wireless network metadata for our crowd-sourced location database.
You can find Mozilla Stumbler on Github to contribute to its development.
Firefox for Android
Firefox for Android includes a way to contribute data and enhance this service. Enable this feature by opening the Settings menu. Open the Privacy section, scroll to the bottom to see the Data Choices. Check the Mozilla Location Services box.
Firefox Desktop
Firefox Desktop allows you to use this service instead of the default location provider. In about:config change the geo.wifi.uri setting to:
https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%