Halo Custom Edition
From Halopedia, the Halo wiki
Halo Custom Edition, sometimes abbreviated Halo CE (which is also often accepted as an abbreviation for Halo: Combat Evolved), is a free expansion that requires a key code of Halo: Combat Evolved by Bungie Studios, which was ported to the PC by Gearbox Software. Halo CE was released by Gearbox as an unsupported version of the original game. However, users do not need to pay to play it, as it is downloadable from selected websites.