Join.mp4 Direct

A free, open-source tool for Windows and Mac, found at mp4joiner.org . It joins files without re-encoding, preserving original quality and working quickly.

A highly recommended tool, often preferred on Reddit forums for merging without quality loss. Online Tools (No Installation) Join.mp4

A popular online merger that allows you to drag-and-drop clips and add crossfade transitions. A free, open-source tool for Windows and Mac,