Cycles Render Engine
Cycles is a new render engine available since Blender 2.61. It is still under development, and intends to become a render engine with a focus on interactivity and ease of use, while still supporting many production features. Developer documentation is also available.
Cycles is bundled as an addon that is enabled by default. To use Cycles, it must be set as the active render engine in the top header. Once that is done, interactive rendering can be started by setting a 3D view editor to draw mode Rendered. The render will keep updating as material and object modifications are done.