A free, open-source first-person shooter for the Quake engine. Every map, model, texture and sound is original and BSD-licensed.
Mouse to look, WASD to move, click to fire, Space to jump, Esc for the menu. On a phone: left thumb moves, right thumb looks.
Choose pak0.pak (and pak1.pak for the full game) from the id1 folder of your Quake install. No install? id Software's free 1996 shareware episode, quake106.zip (9 MB), from a mirror of id's FTP site, works too: choose it as downloaded. Files are read in this tab and never uploaded.