Half-Life is one of the staples of first-person shooters, and one that no PC gamer should go without. While I do not believe the original game aged too well, Half-Life: Opposing Force is a game that I must recommend to anyone looking to play the series. This goes doubly for anyone looking to play a great first-person shooter on an old PC system.
While the story is a bit campy nowadays, and the graphics have certainly aged, the game still has solid gameplay and is perfect for those looking for a budget buy on a bare essentials PC.
Look at that all that glorious low-end graphics goodness...