Mac OS X Server 10.4.7 Tiger (Build 8K1079)

Released 2006-08-07 alongside the original Mac Pro.
$499 for 10-client license, $999 for Unlimited-client license.

It is the first Universal release of Mac OS X, meaning one disc will install on both PowerPC-based and Intel-based Macs.
Notably, it was one of the first versions of Mac OS X to come in a smaller, approximately 5" square box.