Xerix 2: The Caverns of Mars

Xerix II is the sequel to the original Xerix game. It contains the same impressive parallax scrolling as the first game, with the addition of multiple different enemies to fight. There are weapon upgrades to collect as well as a nice soundtrack. It plays decently but it's not especially remarkable beyond its scrolling and a few other features and options. (As per the Classic DOS Games website this game was made freeware by its author, even though this registered version of the game itself still says "do not distribute" etc.)