Here are the web-based spreadsheet applications I’m aware of:
Google Docs & Spreadsheets
This spreadsheet comparing EditGrid, Google Spreadsheets, iRows, Numbler, NumSum and Zoho Sheet has been created in each of the following applications, giving you yet another way to compare them:
WikiCalc, iWoorx, and a video of interest to Web 2.0 enthusiasts
And here’s an interesting video about iWoorx Wiki Spreadsheets (built on WikiCalc).