Well this one may need its own spot but will post here. It is Google gears an add-on to that lets you know at least before it adds it.
This could have use in games. I think i seen a old post on here some time back about having the user store images and other things on the computer I will be looking in to it a bit more.
Gears is an open source project that enables more powerful web applications, by adding new features to your web browser:
Desktop Let web applications interact naturally with your desktop
Database Store data locally in a fully-searchable database
WorkerPool Run JavaScript in the background to improve performance
http://gears.google.com/