Now that you know a lot about how canvas works, lets explore what it's actually good for and some useful libraries.
RGraph is a free for personal use charting library for canvas. It has many different chart forms.
ZingChart is a hosted charting library with a visual builder. It renders in many different output formats, including Canvas, and can handle large datasets.
Wolfenstein 3D recreated in canvas.Opera Dev Article
Akihabara Game Engine
ImpactJS: fast commercial game engine
Pirates Love Daises is a tower defense game done entirely in canvas.
Muro: Deviant Art's webbased painting program.
SketchPad: another drawing program with a very classy UI
Ben Joffe's canvas font script. Converts a font on your computer into an image which can be rendered with canvas. This lets you use a custom font on computers that don't have that actual font installed.
A canvas enriched children's poem. The text is markup and the graphics are in a transparent canvas.
Josh On Design
EaselJS: A graphics library loosely based on Flash's display list. Easel JS
canvg: an SVG renderer built with canvas
Pixastic is a photo editor and image processing library. It has tons of Photoshop style filter effects
Hype by Tumultco, a commercial drawing and animation tool which outputs straight HTML 5
Leonardo Sketch: open source drawing tool which outputs to canvas and Amino code, among other formats. It is extensible and has some neat social features.