The Marathon Trilogy is the spiritual "predecessor" to Halo and was created by Bungie between 1994-1996 originally for Mac (Durandal was originally the only Marathon ever made for PC).
Download Marathon 1 (Windows)
The story begins with you playing a security officer known as "The Marine", a cyborg sent to the UESC Marathon. On Marathon you must defend the ship from an alien race known as the Pfhor, who plan to enslave the people onboard. Marathon has twenty-seven levels and can be somewhat surprising/scary.
Download Marathon 2: Durandal (Windows)
After being in stasis for 17 years, the rampant AI Durandal awakens you and you assault Lh'owon, the planet of the S'pht, who were enslaved by the Pfhor. The Marine is sent in search of finding something that will help free the S'pht and beat the Pfhor.
Download Marathon Infinity (Windows)
In Marathon Infinity, the Marine jumps through alternate realities and tries to prevent an entity from being released from its prison in Lh'owon's dying sun.
Despite the game's old age, the graphics are decent compared to other games during the time. It also features a Deathmatch mode you can play with other players. (Oh, and use Marathon Infinity for Multiplayer. It has all the maps. If you use the other two the chances are it may crash because it didn't have a certain map!)