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.
(I couldn't find a SAK 3.0 logo, so that's why the win2k is on the article ;) )
“Windows-based Server Appliance” means an industry or task-specific device running Windows 2003 (as opposed to a "General Purpose Device," described below) where the device's base functionality, and the extent to which that base functionality can be enhanced after manufacture, is limited and fixed at the time of manufacture. Such limitations on after-manufacture enhancements include both hardware and software enhancements. The limitations on the device as manufactured must not allow the product's base functionality to be significantly modified. An example of such a device is a storage server device for storing data. For task-specific devices, a manufacturer may provide after-manufacture enhancements to the software to fix defects, improve performance, or enhance the base functionality. An example of an enhancement to the base functionality for a storage server appliance is a module to interpret a new file sharing protocol. A manufacturer may not provide after-manufacture enhancements that turn the task-specific device into a General Purpose Device. A “General Purpose Device” is a device that provides, in place of or in addition to, task-specific functionality, general purpose server functionality that can be easily modified by the end customer to add hardware, applications, or enable services that add complete new functionalities to, or in place of, the task-specific functionality that was implemented at time of manufacture.