The GBC (or Great Ball Contraption) has always been a favorite of mine. It's something I helped start, and has been embraced the world over. Recently there's been a new series of videos by YouTube user akiyuki showing some amazing mechanical solutions, but one just popped up that shows a robotic arm (driven by an NXT) that is simply amazing to watch:
(earlier portions of this are here, and are absolutely amazing considering that there is no NXT or electronic intelligence involved. Another great mechanical implementation is here).
Mechnically (and, clearly, in terms of programming) this individual has done an amazing job. But even beyond that, they have a look that inspires. Too often my robots are functional, and that's neat... but taking the extra time to make them look really good, that attention to detail, is rare. Here is a user that manages to combine all those facets... and does it really well.