One of the best cloud based apps that I know and use is Evernote. It is a bit like Microsoft OneNote in that it allows your to keep notes and then have those notes sync’ed across a number of different devices.
Personally, I initially used Evernote to sync study information between my different Windows PC’s initially. However I soon graduated to using it on every device, including the web and iPad. One handy application of Evernote was when I wanted to take a screen capture of a customer’s machine I could do so directly using the web based version of Evernote. Once that was done I knew that when I returned to my office that screen shot would be synced with my desktop. Very, very handy.
The one place that Evernote was missing from was Windows Phone 7 (which I adopted recently). This made me wonder whether the Windows Phone 7 platform had a future simply because an app like Evernote is everywhere else. However, have no fear, Evernote for Windows Phone 7 is now available from the marketplace and like on all other platforms is free.
If you have never looked at Evernote then I strongly recommend you do as it is truly a great example of a cloud based application and besides that I have found that it can really improve your productivity.