Sega Master System / Mark III / Game Gear
Very much in the Operation Wolf mould, this shooting game requires you to blast a variety of enemy sprites with your crosshair sight as you make your way across levels of horizontally scrolling landscape.
Although it seems a bit on the dull side at first, it's actually pretty good fun and grows on you the more you play it. The game difficulty is set on quite an easy level, making it very simple to get into and very addictive in the short-term.
The lasting appeal is questionable, but if you're after a shooting game which doesn't require a light gun, this is worth taking a look at.