Dabble DB is the most practical business-related Web 2.0 application I've seen this year. It allows you to copy data from MS Excel into the web application and manipulate that data in ways you can't do easily in Excel, including turning that data into a more easily managed relational database. Furthermore, the service makes it possible to share the data easily with others via RSS and iCal.

If you use Excel a lot, you'll want to check out the demo video. Note that this is not a free service.