Nick Drake, especially the album Pink Moon which is just him and a guitar, very good album, no, amazing album.
Medeski Martin and Wood. Very good jazz/jam band.
G. Love and Special Sauce, mixes jazz, blues, rock, country, rap and R&B for a pretty cool sound.
Ben Harper, very good artist, big in Europe, not so big here in North America. Gets too preachy some times(like his newest album).
Jack Johnson, newer artist, has played with both G Love and Ben Harper. A surfer from Hawaii(top ranked, too), his music is more layed back than previous two.
If no one has picked up on them yet, Coldplay is a good ensemble band. I know, i know, they ae all over the radio, get played too much, etc. Reminds me of Pink Floyd and Radiohead mixed together.
Portishead. Unique, to say the least.
Last but not least, Morcheeba. There first album, Who Can You Trust?, is a very mellow piece. The lead singer has a very addicting voice. Their second album, Big Calm, is much 'happier.'