A vertical spinner battlebot using two full battery packs as a weapon driven by 8 old (really old high rpm) motors. Brutal.
A marble sorter that I think is particularly elegant:
MSG or Mobile Stuff Grabber, a ROV that could pick up and transport small items around the house:
A lego incarnation of the SHRIMP platform, Version 1 (it did get completed, with NXT bricks etc. but had the unfortunate tendancy to fall over sideways if it just one side went over an object, and so I built version two)
And finally, the largest mobile Lego thing I have ever built, a massive bidirectional sumo bot driven by 12 of those old lego motors I mentioned earlier: