Classic three-dimensional first-person shooter Descent and its sequel Descent II are now available on Steam for $6.99 and $9.99 respectively. Originally released in 1995 for MS-DOS, Descent puts you in command of a ship as you navigate labyrinthine environments and destroy virus-infected robots. Its successor (which was planned as an expansion) runs on the same engine, but features new weapons and better robot AI.
Windows XP or above is required, so if you use any other operating system, the Descent series won’t work on it. Alternatively, you can download both Descent and Descent 2 for $9.99 on GOG and the bundle comes with Windows and Mac support.