- The last part of a good tutorial (also check the comments for a list of other projects): http://codetheory.in/different-tools-for-our-sketching-application/
- Simple pure Javascript implementation: https://bitbucket.org/vishnusathyanesan/my-paint-application
- SVG Edit - a complete graphics editor in the browser: https://code.google.com/p/svg-edit/
- Paint board - ugly but works pretty well: http://gospodarets.com/developments/paint-board/
- Canvas Web Paint - a pretty cool swatch-saving feature: https://github.com/boringmachine/CanvasWebPaint
Overall I think the best-executed lines are in the first link - slightly anti-aliased and with no vector-jitter.
Resources for Wacom pressure-sensitivity: