Many, many good websites for bluegrass but one that I have persnally enjoyed is acousticdisc.com.
A few that stand out are:
THAT HIGH LONESOME SOUND
Old and In the Way
This is a very cool and young all star band that includes Peter Rowan, David Grisman and Jerry Garcia. Despite these musical giants, the musician who stands out and I begin my worshipping, is Vassar Clements. I get the sence that the other band members know that Vassar is in the zone, like a basketball player with the hot hand, and they set him up and get out of his way. Just a great band with phenominal musicians who don't let their egos get in the way.
Everything with Grisman and Garcia is excellent.
"Doc and Dawg" Doc Watson and David Grisman... need I say more?
Not exactly all bluegrass, but two more suggestiosns who like music of this ilk are: The whole tone poem series and "Stephane Grappelli and David Grisman Live."
Lots more good stuff at this site.