a recent(ish) New Scientist that a New York violinist, Mari Kimura, can get notes one octave lower than normal out of a violin's G string using a special technique she discovered (it's easier or better with older, more used strings), in other words she can get cello-like sounds out of a violin. She's called this "subharmonics".
I found Mari Kimura's webpage (new page, old page for historical interest) and had a listen to some MP3 excerpts from pieces she's composed to demonstrate subharmonics, where you can hear those interesting octave-below sounds, e.g. Caprice for Subharmonic 3rd (there are a few more extracts on that webpage), and she's even produced a CD of her subharmonic works. (UPDATED for Mari's note about the URL change, thanks Mari!)