[jahozer]

Frank Zappa
Ernest Ranglin

[gratephulphish123]

led zeppelin. plain and simple led zeppelin songs are fun to play, catchy riffs too

[Henry]

Two I'm having fun with lately:
Jimi Hendrix - the bluesy stuff
Harvey Mandel's first solo albums

[astroman99]

Los Lobos, Rockpile, stax records, Dr Feelgood, Gram Parsons...

[Django]

I agree with Los Lobos, rootsy and R&B ish at the same time. I am asumming you mean to jam along with as if sitting in, in that context Allman Bros. leaves me a little cold, it seems like unless you want to put in the time to learn the actual lines you're too 'outside'.
But guys, Zappa, Phish, Hendrix! You are much better than me if you can jump in with that unprepared.
I agree with Hawaii about blues as a foundation, I got alot of milage out of a T-Bone Walker lesson. Alligator has a good compilation that let's you try many different styles and the 'Deadicated' rain forest album lets you still 'play Dead' but with differant styles.

[guilhermeful dead]

Try Os Mutantes, Novos Baianos and Chico Science & Nação Zumbi.

[sarraqum]

Black Keys are fun to play, so are the Doors,
Someone mentioned Phish but I don't think their "simpler" stuff is that fun to play. I've been learning some of Phish's stuff lately, mainly Landlady and Divided Sky but I am not good enough to pull them off flawlessly yet.
But yeah, try covering a broad range of genres, its the best way.