GB Studio is “A free and easy to use retro adventure game creator for your favourite handheld video game system”, by which they mean, but don’t want to name specifically, Nintendo’s GameBoy.
What a fun idea from developer Chris Maltby. You can output ROMs for emulators, play them on actual GameBoy hardware with a flash cartridge, or even export them for the web (which will even work on phones). It’s a remarkably polished IDE.
Get it here.