eXpresso is a powerful new way to manage and share your Excel Spreadsheets. A real time collaborative environment for Excel spreadsheet users – all available online. Its a secure, browser-based resource that enables users to share and collaborate on any Excel file, regardless of location.The solution assists in the trend toward managed on-line business communities.

eXpresso is complete and immediately usable without installation, support or training. Individuals can use it ad hoc with real-time collaboration capabilities and version control. Enterprise users can rely on it as their corporate standard that meets established benchmarks for security, permissions and backup.

Unfortunately, It does not support Firefox .. or Opera or Camino so if you are on a mac or linux you can’t use it. :/

Another addition to the online office application list , Google Spreadsheets, xcellery, iRows, editGrid