Q&O links a cool article from the London Times on aerogels, apparently the least dense substance manufactured by man. They are apparently great insulators and can be apparently be tweaked to be selectively permeable or absorbent of various substances, making them useful for filtering applications. And they are being used in tennis rackets (I have a theory that tennis and golf equipment manufacturers are to new materials what pornography is to new digital distribution mechanisms -- they seem to always be early adopters).
Postscript: Of course we get this same article about material X every five years ago in the press, so it is OK to be skeptical. But the picture is still wicked cool.