The LEGO Death Star is second only to the awesome LEGO Star Destroyer kit. This is one LEGO fanatics account of building this behemoth complete with lots of photographs.
This 15-inch tall R2-D2 obeys simple commands like "turn around" and "move forward." The little guy responds to 40 commands and moves around without breaking things thanks to sensors. MSRP is about $120.
Ghetto site, but some really cool playsets like the Imperial Gun Tower, Death Star playset and more (he also does custom figures if you check the main page fo the site).