Boktai. A trilogy of GBA games about a vampire hunter who uses a solar gun to fry the undead. The cartridge has a UV sensor to detect actual sunlight to charge up the gun. It's such a silly gimmick but it's used really well.

Sadly the third game never got localized, and although a translation patch does exist it's just not the same without the original hardware. There's a fourth game on DS which did get localized, but they rebranded it as Lunar Knights, excised most references to the original trilogy, and even cut a good chunk of content. It also ends on a sequel hook that will never get resolved.

It's sad to me that we'll never see games this experimental ever again.