At work I lately was building a TYPO3 backend module to control some static HTML generating export script. I wanted the module to look native and sifted through the backend to find the UI elements I needed - which was cumbersome.
Thanks to the helpful people in the TYPO3 chat I was directed to the Styleguide extension.
It provides a list of all backend UI elements available in TYPO3 and was very helpful. It is important to install the git version, because the TER version was outdated.