<p>I want to let the user draw on an image in a browser. In other words, I need <em>both</em> bitmapped graphics and drawing capabilities, whether vector or bitmapped.</p>

<p>Canvas looks good, but is not supported by IE, and though there is ExCanvas, I wonder if ExCanvas is stable enough for consistent use in IE6 through 8.</p>

<p>Or best of all, is there an open-source image/drawing library that supports all this out of the box? I found two dozen or so Web-based image editors or drawing tools, but none support the requirements. (And I'd like to avoid Flash/Flex/Silverlight/JavaFX.)</p>

Even though you said you'd like to avoid it I'd suggest Flash. You could easily use Flash 6 or 7 and these have a > 90% adoption rate. I'd be surprised if you could get that level of support with JavaScript. Flash is truly write once run anywhere, which will cut down on your development time.