Apps with 'Extensive customisability' feature

QTTabBar is an extension for Windows Explorer that brings tabbed browsing to Microsoft's file browser.

REAPER is a complete digital audio production application for Windows, Mac and Linux, offering a full multi-track audio and MIDI recording, editing, processing, mixing and mastering tool set.

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).

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.