A video app with a hard six-second limit that produced more genuinely funny content per minute than anything since.
The constraint was the whole thing. Six seconds forced a setup and a punchline and nothing else, and the looping made the timing matter.
Twitter bought it, then shut it down in 2017. An enormous amount of the internet's comic vocabulary comes directly from it and is still quoted daily.
Cost back then
Free
Goes for now
Archived compilations get millions of views