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Ubuntu JeOS, (pronounced "juice"), is an official derivative of Ubuntu. It is described as "an efficient variant configured specifically for virtual appliances".
Its first release was Ubuntu JeOS 7.10 (Gutsy Gibbon), and the latest release is 8.10 (Intrepid Ibex). It is a concept for what an operating system should look like in the context of a virtual appliance.
Specifications:
99 MiB ISO image
Linux 2.6.22 (7.10) or 2.6.24 (8.04)
Optimized for VMWare ESX, VMWare Server
Intel or AMD x86 architecture
Minimum memory 128 MiB
No graphical environment preloaded as it is aimed at server virtual appliances