EditGrid is the new kid in the Online Spreadsheet Comunity. It offers whole (almost) set of MS Excel features and more: allows multiple users to work on the same spreadsheet simultaneously, uses web browser as user interface, allows to do export/import in many formats, offers API to use of EditGrid in your application.
Other online spreadsheets:
Here is a comparison matrix between EditGrid, Google Spreadsheet and WikiCalc.
This kind of applications are good and useful for the individuals and small companies. But there is a problem of privacy (I’m talking about services similar Google Spreadsheet). When you store some sensitive information on the server of third part company you should trust it. Or you should be ready that your documents will play against you.