I use the Littmann Cardiology III, it works very well. If you buy a Littmann stethoscope from scopesplus.com, you get a free Student Kit, with ascultation sounds, a free Littmann ID tag, and an extra set of earbuds. The CD is a little dry, but it lets you hear a variety of sounds.
The Littmann Master Cardiology is nice too, a bit more expensive, though. Quite a few nurses at work use Prestige Medical ones, those seem to be nice too.
just my $0.02.