Let’s revise some of the most remarkable scenarios.
But Flash comes to the rescue, with some really stunning implementations. It’s worth mentioning open source implementations like Fancy Upload (MooTools), jqUploader (jQuery) and SWFUpload. Flickr’s new upload also takes advantege of this method.
Photo editing / Drawing
The main reason Javacript falls short here is its limitation to come up with most effects, which is the reason js-based editors will do that server-side, adding a delay and thus hurting the user experience.
The only sure way to play a sound file in a crossbrowser, crossplatform way is nowadays Flash.
Persistent Data Storage
There might be certain scenarios where you’ll want to store large chunk of data in the client side (it could be for caching purposes, or for offline retrieval, for example. Flash is one of the most reliable ways, allowing you to store up to 100k seamlessly, and even more if the user authorizes it when prompted.
See dojo.storage for a nice implementation and proof-of-concept
Note: i’m currently working on a MooTools implementation of this.