The OpenVZ project has released a beta kernel based on 2.6.15 (their current stable kernel is based on 2.6.8). Kernel 2.6.15 brings in some new features like CFQv2 I/O schedulers, enhancements in memory management, ext3 online resizing, and so on. This kernel is already stable enough to be used for testing/evaluation. Some things such as 4GB split and Fair CPU scheduler are not yet ported, but they are working on it. For interested kernel developers, they have also released broken out patch sets for the 2.6.15-based kernels, available from here.