Morse [official site] is a wonderful take on the classic Battleships where you'll use actual Morse code to thwart waves of enemy attacks. Set in World War 1, it'll have you defending the land from a three-pronged enemy invasion: by land, sea, and air. All you've got to do is get your own troops past the foe's trenches at the other end and victory is yours. By tapping out Morse messages. No pressure, soldier. This video playthrough explains how it all works:
Morse is a really neat idea because, aside from teaching you actual Morse code, it's more complex than first meets the eye. While the three plains of land, sea and air present their own distinctive threats, they're also interlinked. Land is the most important field as that's where you'll send your troops into No Man's Land, win points, and, ultimately win the game. But infantry on land is delivered by sea, so guiding your vessels safely from end to end there is essential. And air provides recon, so sending planes over the aerial threshold makes the rest easier to pull off.
As you might expect, all of the above can be turned on you, thus any strategy I attempted to employ under pressure regularly broke down, degenerating into a stramash of button-bashing and curse words. It's great.
Cooler still is the fact that game was born of a makeshift DIY job. Creator AlexVSCoding says this on the game's website: "Morse's origins came from developing a Morse code controller earlier this year using a clothes peg and a plank of wood." Here's that very peripheral:
Morse is free and can played online here.