Tweakybeat for the iPhone. One of the first music apps to come out on iOS. I bought it before I had an iPhone - to play with on the test phones at work.
It’s a monophonic sequencer with a single noise source which can be tweaked to make drums, beeps, swoops and gurgles. Such fun and always go back to it for a quick bleep session ever so often

My cat, Skippy had terrible seizures, throwing herself around the room. It was terrifying.
Turned out it was her flea collar. Heard of other people who’ve experienced the same thing