The largest
1 of the "
Enterprise" series of
Sun computer systems.
Can house up to sixteen system boards that can be grouped together logically into seperate systems called "domains". System boards can be added or removed from a running system with no downtime required.
Each system board can contain up to four CPUs, four gigabytes of RAM as well as multiple ethernet, SCSI and Fiber interfaces.
Each system board can support "alternate pathing" (AP) to allow multiple paths to single devices in the event of component failure.
An E10k can therefore be used to create an enormous domain containing sixty-four CPUs and sixty-four gigabytes of RAM, or sixteen seperate domains.
An E10k relies on another machine, the System Service Processor, or SSP. The SSP contains all the configuration information for each domain as well as the boot PROM image for each, acts as a system console for each domain. It is also responsible for booting each domain.
This is not a computer you'll find in someone's study.
Update
Sun have announced the successor to the E10k, the Sunfire 15000. This new machine is reportedly capable of 15 dynamic domains, a terabyte of RAM and about 160 CPUs.
1 This writeup is now very out of date and will be updated shortly. The Sunfire 25K is now the largest system made by Sun Microsystems.