There's been a lot of buzz about the new Google Spreadsheets, which were announced yesterday. The product will allow people to collaborate online, and is currently being beta tested. I'm not sure if there are still slots available to get in on the limited test, but if so, you can sign up for an invitation here.
A quick look at the technical specs: the limited version currently being tested allows the creation of up to 100 spreadsheets, which may have up to 20 tabs, 50,000 cells, 256 columns and 10,000 rows. Spreadsheets can be imported from Excel and CSV formats, and can be exported to those same formats (and HTML, too).
It supports the most common functions in Excel, including the most commonly used mathematical, statistical, logical, and financial functions. However, at present, it does not have any graphing capabilities, and you can't change formats on a group of cells by highlighting and right clicking.
All in all, it's definitely worth checking out. For working on simple spreadsheets in collaboration, it's got potential (you can chat simultaneously while working on the spreadsheet). And, since it's a web-based application, it can be upgraded continuously without having to go through the hassles of having to be reinstalled each time a new version comes out.