ED4Sim is a spreadsheet tool designed to assess uncertainty in petroleum reservoirs using reservoir simulation. It is a very powerful spreadsheet, that has state of the art capabilities in an easy to use flexible package.
This spreadsheet was developed mainly while working at Nexen, and along with the RSMPlots spreadsheet, the company has allowed it to be released publicly.
Update June 7th, 2016
The link to the Essential regression add-in is broken and I am unable to find this tool elsewhere. This is surprising as I consider it the best free regression tool and as good or better than commercial (not free) tools. I have provided links below to my copies of this tool.