How about a combined backup/sync solution for Excel spreadsheets? The sync portion could work like Dropbox, but the daemon would only pay attention to Excel files on a given machine, or in a folder, like a user's home directory. There would be a company account on a website or a locally running web appliance, where users could share spreadsheets, particular pages in a spreadsheet, or even particular cells. This would be combined with an Excel macro that can pull-down such shared data from the account website. Updates to data would only be sent when the spreadsheet is saved and uploaded by the local daemon.
This could even be automated by screen-scraping Excel running on virtualized Windows instances.
A automagically simple syncing solution for excel data ala dropbox would be great and a hell of a start. This might even be a nice app for the dropbox api.
It wouldn't only be a great place to start. Once everyone's spreadsheets are backed up, you automatically have a repository of all of their data, which can be federated, shared, synced, data-mined, displayed in a web app, etc...
Doing this with the new open source dropbox thingy Sparkle would be great. I like it because it doesn't force people to switch from Excel, but adds another awesome layer: web access, versioning, syncing,etc.
This could even be automated by screen-scraping Excel running on virtualized Windows instances.