I wanted to make something using Server-sent events, so I made a silly drawing app.
This is a simple, collaborative drawing app. Everyone who is on the page shares the same screen. When someone adds a point, it shows up for everyone else. There's three rendering modes, all of which share the same data, so someone can make a drawing in one mode, and the other modes will also change.
Anyway, you draw by clicking on the screen. The longer you hold the mouse button down, the stronger the point is, depending on your particular mode. It's random, and rough around the edges, but fairly entertaining.
It runs on Sinatra, and uses p5.js which is similar to processing.js.