use www.cellmaster.com.au (disclosure I am the founder) At the moment it is free for low volume usage.

These also look like they would do what you want, maybe spreadsheetconverter depends on your needs:

BaseCase.com

stoic.com

www.spreadsheetconverter.com

www.smartsheet.com

http://www.forguncy.com/ (Japanese)

www.spreadsheetweb.com

Please tell me more. Details of my email on www.cellmaster.com.au.

P.S. I also like ethercalc as spreadsheetguru suggested - but think many of above would be less work.