The following graph shows the install paths for SharePoint:

The version of the current installation can easily be looked up in the Central Administration: Operations > Topology and Services > Servers in farm.

Please find the download links and further information on this excellent Microsoft site: http://technet.microsoft.com/en-us/office/sharepointserver/bb735839.aspx .

Thanks to Penny Coventry for his list of version numbers which was a good start for this.

UPDATE: Added Feburary 09 CU.

UPDATE: This Post is now obsolete and won’t be updated further. Please refer to the new version table post: http://www.fkollmann.de/v2/post/SharePoint-version-table.aspx.