Build2 is made up of a build system, package manager, and web repository interface. The Build2.org project site explains, "The aim of the project is a modern build system and package manager for the C++ language that provide a consistent, out of the box interface across platforms/compilers. Having a sane build file syntax would be nice, too. The toolchain is written in C++11 and while not strictly C++-specific, that's the primary focus."
Boris Kolpackov of Code Synthesis announced the project to the GCC developers today due to Build2 having its first alpha release, which is considered "more of a technology preview than anything that is ready for production." So far it's been tested on Linux, OS X, and FreeBSD. There is no Windows support yet but there is cross-compilation support.
More details on Build2 can be found via the Build2.org documentation.