Head to the Official Website and download the realtime viewer. System requirements are high, so i would not advise that you download it unless you have at least a Gigabyte of RAM, a 2 Ghz CPU and a 128 MB video card.
For those of you who do not meet the system requirements, i've made a video showing off physics, collisions, first-person walking mode, etc.. It's 6:30 and encoded as usual in Divx4 (50 MB):
And here is another video, low quality version (25 MB), showing off things a bit differently (the beginning is the same though): Smaller video