Epic has released the December 2009 Unreal Development Kit (UDK) Beta.
UDK is the free version of Unreal Engine 3. UDK users get full access to the UE3 toolset and all UnrealScript source code.
Significant new features in this version include DLL binding, ActorX support for 3ds Max 2010, the visual-only InteractiveFoliageActor, navigation mesh additions and much more.
Build upgrade notes are available at the Unreal Developer Network (UDN).
In addition, our engine team maintains a Content Blog that summarizes useful additions and improvements to the engine.