Well in the latest episode of the ebook wars, Amazon has gone cloud. To avoid Apple taking a 30% cut off every book they sell, they have created an HTML5 app to circumvent the rules. Remember last week, Barnes and Noble simply removed the store link from inside its reading app. ReadWriteWeb has the story:
Amazon Releases Web Based HTML5 Cloud Reader
The kindle cloud reader supports Chrome and Safari (all the WebKit browsers) and does have off line support apparently. I have no time to try it out right now, but I'm interested see how it works.
Kindle Cloud Reader