I missed these when they were released last week; The Black Mesa Media youtube page has been updated with some early alpha gameplay / test footage - Note is does not reflect the current state of the mod.

This one is probably my favourite half Life map:

httpvh://www.youtube.com/watch?v=P-kyqwyzLms&feature=channel_page

httpvh://www.youtube.com/watch?v=UoFSLfWafH0&feature=channel_page