Here are a few things by Danny Thomas

Poi Random Simulator (spins random patterns)

Double Staff Random Simulator (spins random patterns)

Poi Simulator (capable, interesting interface but hard to use)

Poi Simulator in javascript, less pretty but easy to use

3D Connect 4 Game (basic graphics, but difficult to beat)

Interactive lines (drag mouse/finger)

Pretty interactive circle pattern (move mouse)

Harmonograph patterns (use arrow keys)