I've never used MS Sharepoint but are you talking about
something like this...
http://www.google.com/googlespreadsheets/tour1.html
If you do find a freeware, please let us know!
I guess one alternative might be to have your users make
their changes locally
and then upload their changes to the webserver. You would
then write code
to read/parse/datasource/etc this .xls. Or you could leave it
as is and the next
time someone wants the file, send them the version that was
uploaded last.
I know it's a really ugly hack so please respond back with
how you settled on
the problem.
Good luck!