Way back in December, I wrote about Num Sum, a Web-based spreadsheet. Since then, Num Sum has matured and added quite a few features, including the ability to work on Num Sum spreadsheets offline, which is very clever.
Num Sum was one of the first Web 2.0 spreadsheets (I don't like the term "Web 2.0" much but you all know what it means so ...) but it certainly wasn't the last.
One of the coolest features of EditGrid is its Real-time Update (RTU) feature, which allows multiple users to see dynamic changes to a spreadsheet. EditGrid also includes a remote data update facility that can periodically retrieve live financial data on the Web ("includes stockquote and forex") and store it in a spreadsheet.
EditGrid provides more than 500 functions such as SUM(), STDEV(), COUNTIF() and VLOOKUP(), and a future version of EditGrid promises to include functions unique to EditGrid. You can import and export common file formats such as Microsoft Excel, CSV, HTML, Gnumeric, Lotus, OpenOffice, etc., assign fine-grained access control, and define permanent links so that you can refer to your online spreadsheets.
EditGrid operates cross-browser, includes a "post-to-blog" feature and even provides a SOAP-based API.
A great example of the use of EditGrid is the "EditGrid v. Google" spreadsheet that compares the features of EditGrid to Google's Spreadsheets, Microsoft Excel 2003, and the alpha version of WikiCalc.
I used EditGrid for some time and it works amazingly well, appears to be robust, and is currently free.