I got a pretty awesome shareware PC game in a box of cereals many, many years ago. I can't for the life of me remember the name, but, it was a vertical shooter, like 1943. You could choose a character when you started a new game, and had a few missions to choose from. Between missions you could buy upgrades from money you earned in the different stages, like shields or different weapons. I only ever played like 3 levels of that game, I think that's as much as you were allowed to play without buying it :p