It depends.

The only other widespread solution are Java Applets, which are better in some ways (e.g. performance) but worse in others (e.g. start-up time).

Not if you want it to run in a browser.

I guess Canvas is out of the question, if you want folks on IE to be able to use it.

There are canvas emulators that make Canvas more or less functional in IE, albeit with a speed hit. But even then, Canvas isn't designed to do the same things Flash excels at: vector-based animation.

True - for that, you could use scripted SVG.