CyberTracker is a new mobile data collection app for KoBo. It runs on Android and iOS. There are also desktop emulators for PC and Mac. We are building out the [documentation and samples here] (XlsForm extensions | CyberTracker Wiki). You can also get the gist of it by watching this video.
CyberTracker was developed for wildlife conservation which has uncommon requirements. For example, field rangers may not be literate or have other needs like location track timers. Nowadays it is a more general tool, which can collect data for multiple backends.
Customizations are done via the XlsForm extension mechanism. If you work at the XlsForm level, then it is quite easy to try out. There are tutorials and demos.
We do not have full support for all XlsForm features, but most field types and common cases work well. If you are interested in the source code, here is the GitHub repo.
Feedback welcome, thank you!