Blender 2.66

Other new features include Cycles hair rendering, support for high pixel density displays, much better handling of premultiplied and straight alpha transparency, a vertex bevel tool, a mesh cache modifier and a new SPH particle fluid dynamics solver.

Dynamic Topology Sculpting
Dynamic topology is a new sculpting mode that subdivides the mesh as needed, whereas regular sculpting only affects the shape of a mesh. This makes it possible to sculpt complex shapes out of a simple mesh, rather than just adding details onto a modeled base mesh.

Rigid Body Simulation
The Bullet physics library has now been integrated into the editor and animation system, which makes rigid body simulations available also outside of the game engine. This results in a much simpler workflow and gives more control over the simulation.

UI and Usability
Support for MacBook "Retina" Displays was added, proper DPI scaling in all editors, new high-res icons and transparent tool and property regions. User Preferences and Startup File have been split up, new ways to recover sessions were added and copy & paste of objects inside the 3D View is supported now.

Cycles Rendering
Initial support for hair rendering was added, multiple importance sampling for lamps, preview rendering is available inside the properties editor, new closures and optimizations for OSL, non-progressive integrator was improved and support for packed, generated and movie textures was added.