Difference Engine is an extension to Xen, which is an open-source hypervisor. The paper, titled "Difference Engine: Harnessing Memory Redundancy in Virtual Machines", by Diwaker Gupta, Sangmin Lee,Michael Vrable, Stefan Savage, Alex C. Snoeren, George Varghese, Geoffrey M. Voelker, and Amin Vahdat, won a best paper award in OSDI'08. I will not present the conference talk (which is just a taste, and for which slides are unavailable) but the full paper, including my comments on it.