No one has recommended any of the Miles Davis work from the '70's. A lot of that stuff gets way out there, and probably not for novices (unless you're really into funk or Fela Kuti). When you're ready, I'd highly recommend "A Tribute to Jack Johnson" which was originally a soundtrack to a film that no one seems to remember. Probably the best and tightest of the 70's era Miles, and it's a mid-line priced single cd, so it's pretty cheap. It's got some great guitar on it courtesey of John McLaughlin and an uncredited Sonny Sharrock.
Going back a few decades before that, I also really like 'Birth of the Cool', which if I have my Miles history right, is one of the first records to come out under his name (late 1940's?) and was considered pretty groundbreaking at the time. Great stuff seeking new owners in Yahoo! Auctions! http://auctions.yahoo.com
