After one test build and one release candidate, the Blender Foundation rolled out the stable version for Blender 2.66. However, there is no official announcement at the moment.
The developer’s release notes for this build announce the introduction of a new sculpting mode called Dynamic topology, which is designed to subdivide the mesh as you need it instead of sculpting affecting only the shape of a mesh.
This revision also features preliminary support for hair rendering and should handle transparency in images better. Mesh modeling has been improved as well and now you can bevel selected vertices.
Also improved is the particle fluid dynamics thanks to introducing a new particle fluid solver (called Classical solver because it is based on early SPH research), which offers more accurate simulations.
The Mac version of the program brings to the table support for Retina displays. The full change log for this release is available on this page.