Apps with 'Extensive customisability' feature

REAPER (Rapid Environment for Audio Production, Engineering, and Recording) is a digital audio workstation, MIDI sequencer and video editing software application created by Cockos. The current version is available for Microsoft Windows (XP and newer), macOS (10.5 and newer), and Linux.

QTTabBar is an extension for Windows Explorer that brings tabbed browsing to Microsoft's file browser. It comes with a number of nifty features including a favorites function for launching folders and applications, groupings of folders that can be opened together, tooltip...

Nyxt is a new type of web browser for power users. It's built from the ground-up to be completely customizable. Out of the box Nyxt ships with tens of features that allow you to quickly analayze, navigate, and extract information from the Internet.

Doom is a configuration framework for GNU Emacs tailored for Emacs bankruptcy veterans who want less framework in their frameworks and the performance of a hand rolled config (or better). It can be a foundation for your own config or a resource for Emacs enthusiasts to learn...

Tribe is a white-label platform, empowering the new generation of online communities. It boosts users engagement and growth through widgets and integrations to existing stacks.