Sega Master System / Mark III / Game Gear
Originally free with the Sega, and easy available if you check out classified adverts, this cartridge features two pretty jolly little games.
Astro Warrior is an up-the-screen blaster in which you shoot oncoming alien craft, pick up the extra weapons and unleash them on the end-of-level baddies. It's simple, but it's great fun and highly addictive, and is one of the better Sega shoot 'em ups around.
Pitpot is a very original game that offers plenty of laughs - it wouldn't be very good value on its own, but combined with Astro Warrior it's a good second game.
If you can find the cartridge for a cheap price, this is well worth getting.