Via Charles Arthur, some ex-Apple employees have released a Beta version of 280slides. Presentation editor (and presumably viewer) that works in modern browsers. Powered underneath by "Objective-J" which apparently translates down into JavaScript and uses Canvas, SVG or Flash as needed.
Since I've never used the KeyNote app in MacOS, or even Google Presentations, someone will have to give me the skinny on how this stacks up. But it looks nice! Even in Firefox 2 on my 8-year-old Ubuntu machine, the performance is acceptable. I should try it on my Macbook Pro. Check it out.