This is a Exchange 2007 Mailbox Server Role Storage Requirements Calculator spreadsheet which goes with the following blog post:

http://msexchangeteam.com/archive/2007/01/15/432207.aspx

Spreadsheet version updated to 17.6: 06/16/2010

For the list of updates, read this.

Important

Changes made to the calculator in the v11.x time frame have resulted in the calculator no longer being backwards compatible with older versions of Excel.  If you do not have Excel 2007 and would like to use the calculator, you can get the trial here.