Phish and Widespread Panic probably have the best claim to carrying on the Dead's "ethos." They're different enough in how they jam and the resulting feel of their music. The Dead were not the best musicians, but had phenomenal song-writers working with them and a natural ear, and thrived on chaos and disorder musically. Phish, at least originally, were very technically talented nerds whose musical construction was significantly less organic seeming than the Dead - and their jam sessions were much more organized as a result. Compare some of the crazy Dead jams, like Playing in the Band in its oddball 10/4 time signature that morphs out into one (or two, or three or four) other songs and then comes crashing back into Playing, or the 35 minute long versions of The Other One (or 45 minute Dark Stars), with Phish jams and the difference I'm talking about gets clearer. Do yourself a favor and go see a Dead & Co. show while Bobby, Mickey, and Bill are still touring. I know, I know, "but John Mayer...". It's worth it.