If we're mentioning and linking to web based CSS animation tools, let me humbly notify you of Edit Room:

http://www.edit-room.com

I'm the developer, and my development prototype is available to play with at that link. I believe my approach is both unique and will be a better tool for designing and animating in the browser.

Adobe is also taking on this problem: http://labs.adobe.com/technologies/wallaby/

These tools could win out on price, stability or simplicity but if Adobe doesn't screw this up, they I'd put my money on Flash due to it's advanced featureset.

That said, there's probably room for a few players in this space. Heaven knows Adobe needs the competition.