Quick note: Google will make it's applications favor HTML5 over the proprietary Gears. News here: Gears API Blog: Hello HTML5
The "canvas" tag was first introduced by Apple in WebKit for the <a href="http://www.apple.com/macosx/features/dashboard/">Mac OS X Dashboard</a>
Meanwhile, it is supported by Mozilla, Chrome, Opera. The current state of <canvas> allows to do full 2D operations and animations directly in the browser. Tutorials usually start with boxes and circles, but there are advanced examples that show off "oldschool" demo-effects or even the display of 3D objects and landscapes. The <canvas> element is even supposed to provide a 3D context, that shall allow such transformations. However, this is not yet supported on all popular browsers.
Mozilla.org <canvas> tutorial:
https://developer.mozilla.org/en/Canvas_tutorial
Introduction to the <canvas> element:
http://www.brighthub.com/internet/web-development/articles/38364.aspx
Other <canvas> Examples:
http://overstimulate.com/projects/canvas
http://teethgrinder.co.uk/canvas/particles.html
http://ponderer.org/download/canvas_demo/
(via http://www.entish.org/wordpress/?p=283)
http://www.ajaxline.com/libraries-and-examples-for-working-with-canvas