oh man, I've been waiting for a thread like this...
I got into jazz a few years ago, and I can honestly say it changed my life... I started listening to "The Bad Plus" and then moved to the old masters. If you're just getting into it, The Bad Plus is definitely one of the best contemporary jazz bands out there. They do a bunch of covers from stuff you already know, as well as some amazing original compositions.
Here is a little list of some of my favorites... (Oh and with most this stuff, you have to listen to it as a whole album. Don't cheapen this shit by downloading a single song)
Grant Green- Idle Moments, Green Street
Charles Mingus-Ah Um
Herbie Hancock- Empyrean Isles, Maiden Voyage
John Coltrane- A love Supreme
Miles Davis- Kind of Blue, Bitches Brew, Miles Smiles, E.S.P.
Monk- Monk's Dream
If you're into bass man, listen to anything with Ron Carter or Mingus.