As anyone working on different machines know, keeping files in sync between different computers is really a pain. But it’s not only files, its as much the settings, bookmarks etc. that change when you shift to another machine. Not to mention backup. In large organizations this is solved using central servers where your stuff is stored remotely and synced to your local machine when you log in. This works fairly ok, except the whole centralized approach, but its a major pain to set up if you’re a small company or a dude with two machines at home. Especially considering your want access to your data anywhere you have an internet connection.
For a long time I had no great solution for this, but recently a very neat piece of software has surfaced. Dropbox hides the central server away in the cloud and more or less just gets out of your way and lets have a virtual home directory. They provide 2gb storage for free (as in beer), enough to store the most important stuff. and the software works is cross platform (Mac, Linux and Windows).
What has this to do with web apps? For a long time, one of the great advantages of web apps has been the ubiquitous access to your data. And with a virtual homedir you can getmost of that for normal applications as well.