Commander Keen - the hero of old style jump 'n Run. An all time classic and a sure must. This first part of the trilogy "Invasion of the Vorticons" "Marooned on Mars" puts you stranded on what is most likely Mars. You got to find pieces to get your spaceship going again. And off you go wandering the surface of the planet as well as caves and similar places filled with funny looking monsters you got to jump over, jump on or disable with your raygun.
Commander Keen is a classic and it's no wonder that it's STILL being sold after all these years, this part - as well as part 4 - is shareware, so you are free to download it. You can also get it directly from Apogee at www.apogee1.com.