As I promised earlier, here is the result of my comparison of various online spreadsheet services. I compared the services based on the following criteria.
- Basic Requirements
 - Interpretability
 - Usability
 - Functionality
 - Sharing and collaboration
 
Do let me know if I have overlooked something and also your comments and suggestions and I will try to keep this list updated. Also let me know if want me to add some more services to the list.
| Features | Zoho | iRows | NumSum | EditGrid | |
|---|---|---|---|---|---|
| Basic | |||||
| Browser Support | IE 6, FireFox 1.07 | IE 6, FireFox 1.0, Mozilla 1.4 | IE 6, Firefox 1.5, Opera 9 | IE 6, Firefox 1.5 | IE 6.0, Firefox 1.0, Epiphany, 1.8.5, Galeon 2.0 | 
| Stable or beta | Private beta | public beta | Out of beta | Public beta | Public beta | 
| Regsitration | Needed | Not needed | Needed | Needed | Needed | 
| Interoperability | |||||
| Import Excel (*.xls) | Yes | Yes | Yes | Yes | Yes | 
| Import CSV (*.csv) | Yes | Yes | Yes | Yes | Yes | 
| Import OpenDocument spreadsheet (.ods) | No | No | Yes | No | Experimental | 
| Import OpenOffice.org Calc (.sxc) | No | Yes | Yes | No | Yes | 
| Import Lotus 1-2-3 (.123) | No | No | No | No | Yes | 
| Import TeX (.tex) | No | No | No | No | No | 
| Export Excel (*.xls) | Yes | Yes | Yes | No | Yes | 
| Export CSV (*.csv) | Yes | Yes | Yes | Yes | Yes | 
| Export OpenDocument spreadsheet (.ods) | No | No | Yes | No | No | 
| Export OpenOffice.org Calc (.sxc) | No | Yes | Yes | No | No | 
| Export Lotus 1-2-3 (.123) | No | No | No | No | No | 
| Export TeX (.tex) | No | No | No | No | Yes | 
| Export HTML | Yes | Yes | Yes | Yes | Yes | 
| Export to PDF | No | Yes | No | No | No | 
| Print Preview | No | Yes | Yes | Yes | Yes | 
| Max size of imported file | 400 KB | No particular limit | 500 KB | 500 KB | Not available on site | 
| Usability | |||||
| Key board navigation | Yes | Yes | Yes | Yes | Yes | 
| Right Click menu | No | No | Yes | No | Yes | 
| No of sheets open (by default) | 1 | 3 | 1 | 1 | 1 | 
| Undo/Redo | Yes | No | Yes | Yes | No | 
| Cut, copy and paste | Yes | Yes | Yes | Yes | Yes | 
| Line Wrap | Yes | Yes | No | Yes | Yes | 
| Resize Row | Yes | Yes | Yes | Yes | Yes | 
| Resize Column | Yes | Yes | Yes | Yes | Yes | 
| Bold, italic, underline, colors etc., | Yes | Yes | Yes | Yes | Yes | 
| Align Horizontal and vertical | Yes | Only Horizontal | Yes | Yes | No | 
| Insert rows and cols | Yes | Yes | Yes | Yes | Yes | 
| Delete rows and cols | Yes | Yes | Yes | Yes | Yes | 
| Delete Selection | Yes | Yes | Yes | Yes | Yes | 
| Functionality | |||||
| Search | No | Search Files based on tag | Yes | No | No | 
| Add Images | No | No | Yes | Yes | No | 
| Fill Series | No | To be impemented soon | Yes (but Only copy) | No | No | 
| Functions | around 200 | Yes | around 50 | Yes | around 500 | 
| Format Numbers | Yes | Yes | Yes | Yes | Yes | 
| Format Currencies | Yes | Yes | Yes | Yes | No | 
| Header rows and cols | Only Rows | No | Yes | No | No | 
| Hide rows and cols | No | No | Yes | No | No | 
| Merge Sells | Only horizontal | No | Yes | Yes | No | 
| Sort | Only entire sheet, by one column | No | Yes | Yes | No | 
| Charts | Not yet | Bar, Column, Pie, Line | Pie, Bar, Line | Line, Area, Bar | No | 
| Automatic Save | Yes | Yes | Yes | Yes | Yes | 
| Build-it Templates | No | No | No | No | Yes | 
| Publish on other sites | No | Yes | No | No | |
| Sharing and Collaboration | |||||
| Tagging | No | Yes | Yes | Yes | No | 
| Chat | Yes | No | No | No | No | 
| Share read access with another user | Yes | Yes | Yes | Yes | Yes | 
| Share write access with another user | Yes | Yes | Yes | Yes | Yes | 
| Share read access with public | No | Yes | Yes | Yes | Yes | 
| Share write access with public | No | No | Yes | No | Yes | 
| Password-protected read access | No | No | No | No | Yes | 
| Password-protected write access | No | No | No | No | Yes | 
| Real time collaboration | Yes | No | No | No | Yes | 
| Conflict resolution | Last save wins | Last save wins | Not applicable | Last save Wins | Last save wins | 
Links to the above sevices
PS: I composed this list in Excel and then imported it to ZohoSheet and then generated the HTML page and then hand coded and cleaned it.