Metal Storm (NES)

If a cover like this doesn't look great to you, you need to have your head examined!

For those of you looking for a great retro game that you've never played, check out Metal Storm.  The game is really pretty basic: You are a walking armored mech and you have 2 abilities, shooting and jumping.  The really great thing about this game though is that if you hold up while jumping, you reverse gravity and you can walk along the ceiling.  

It may sound like a flimsy gimmick but this game was developed by the same people who made R-Type (IREM) and the mechanic is exceptionally well integrated into the game play.  The production values of this game are surprisingly great as well.  The explosions are animated really well and make you feel a little better about blowing up.  Considering that there's a fairly good chance this is ALREADY on your computer in some NES romset you downloaded, it shouldn't be too much effort to give this game a shot.