ClassyCountdown is a jQuery plugin that allows you to easily create clean circular countdowns. It is fully customizable and includes 12 themes, which can be modified and it uses HTML5 canvas for rendering the circles.
Particleground is a jQuery plugin for snazzy background particle systems. Particleground includes an optional parallax effect controlled by the mouse on desktop devices and gyroscope on mobile devices.
It works in any browser that supports HTML5 canvas.
waterpipe.js is a jQuery plugin for smoky backgrounds generation (HTML5 Canvas).
Sketch.js is an easy-to-use jQuery plugin that allows you to create canvases upon which visitors can draw. It supports multiple browsers including mobile devices (albeit with performance penalties).