A few killer apps I've been thinking about.... Some are partially built.
Online bookshelf. Safari at oreilly.com has the concept but it's missing the feature that would get me to subscribe. I have about 50 O'Reilly press books sitting on the shelf. If I could use ISBN or even answer a randomly selected question from the text and register the print copy for online access I'd subscribe in a minute. The savings in the weight of my backpack and the ability to access the books from anywhere I'm online would be well worth it.
The other is an application that Zoe has a jump on. My wish is for something a little different. First, I'd like a standalone database that I can scale independent of the application. MySQL comes to mind for the ability to do replication. I want each copy of the database, the server, the desktop and the laptop to be able to sync and keep my email archive in sync. Finally I'd prefer to have a separate web server. Apache with PHP and MySQL means there would be a completely platform independent setup for sending and receiving e-mail with an archive that will scale nicely.