This is a Comparison of Spreadsheet Functions between Excel, Google Spreadsheets, EditGrid, ThinkFree - Calc, OpenOffice Calc and Zoho Sheets made in July 2007. It consists of more than 500 unique Spreadsheet functions.
It took some time to compile this list since the quality of the Suppliers Spreadsheet functions lists are far from flawless. Also the interpretation of what category a certain function belongs to varies. Since then I have not attempted an update.
Still, I think it is the most complete Spredsheet function comparison list right now.
EditGrid just took a great leap with the introduction of 41 External Data Retrieval Functions. They now have more then twice the amount of functions compared to Google, and more the 50% more the Excel and the other competitors. Here is one example what you can do with the new external data retrieve functions.