The Ultra KSM (UKSM) patch-set for the Linux kernel continues to be maintained for providing transparent full-system memory de-duplication for Linux.
Several Phoronix readers have been writing in about UKSM in recent months, either asking about it or requesting it be written about and benchmarked on Phoronix. Most of the information on UKSM is unfortunately in Chinese. UKSM is about de-duplication of data in system memory rather than being another de-duplicating file-system. UKSM can work for KVM virtualization as well to reduce memory usage for guest virtual machines and there is also a KernelDeDup project for supporting Xen virtualization too, in an effort to reduce memory pressure.
UKSM's most recent version is 0.1.1.1 from May and back in June the project put up support for automatic tracking of the mainline Linux kernel. There's UKSM packages for Debian Sid, Ubuntu 12.04, Fedora 16, and Fedora 17.