ThinkGeek have a range of nerdy mugs that will satisfy nearly every geek's preference, but this one in particular has to be one of the best. The black mug pictures Darth Vader, and when a hot beverage is poured inside, the words "You don't Know The Power Of The Dark Side" becomes visible and the lightsaber glows as well.
Star Wars
You can get the mug for $12 (£8) on the ThinkGeek store, and with the release of Star Wars: Episode VII in December, these are probably going to be in even higher demand.
Star Wars Mug