SAK 3.0 (Server Appliance Kit) is a tool-set for creating specialized windows servers, like a Windows powered NAS (Network Attached Storage), VON servers, Anti Virus Gateways, print servers, and Call center voice servers.
SAK 2.0 for Windows 2000 also contained tools to build a dedicated IIS 5 web server, this is now available in Windows 2003 server Web edition
So these are windows powered boxes dedicated for 1 function, the most common Windows powered appliances are Windows Storage Server 2003 (NAS 3.0)
This kit is typicly for OEM system builders, not just to install on your company servers or at home, but you can have a go and try to build a NAS yourself.