OpenVZ is a nice working implementation. Unfortunately, it is for specific (older) kernels only. It is an out-of-tree implementation and seems to intend to remain as such.
Linux-CR builds on the experience garnered through the previous out-of-mainstream projects such as OpenVZ and Zap. In developing it we aim to improve the design and extend the range of of supported features. Not less important, Linux-CR aims for inclusion in mainline.
(OpenVZ developer contributed to our effort in the past and we hope for more :)