XXTEA

It's fun to see this, because TEA ("tiny encryption algorithm") was one of the first and easiest ciphers I ever learned to code. I even memorized it! It's been through the mill a few times (apparently it was used — and immediately broken — in Microsoft's original XBox console), so XXTEA ("extended extended TEA") is the current version. The advantage is extreme small size in ROM. Rijndael (AES) is still preferable in most applications — it's well vetted and open source. Simple netdiving can turn up hours of similar fun, especially if you Google Schneier, Threefish, skein and RadioGatún, but life is full to the brim with other hours...

For instance, I always forget how to tune a baritone ukulele even though it's the same as a guitar, so I use a mnemonic: Do Girls Believe Everything ...? The "or nothing" part goes without saying.