even if this topic is really old i want to answer
i LOVE ths underground hip-hop so i can give advices about it.
in this moment i'm listening "little johnny from the hospitul" of Company Flow. this album is just FABULOUS! it's an intrumental album but really really good!
a list of my favourite artists (and i'm sure that i will forget few manes... )
Aesop Rock - Float
All Natural - No Additives, No Preservatives
Analog Brothers - Pimp To Eat
Atmosphere - Lucy Ford: The Atmosphere EP's
Beastie Boys - Paul's Boutique (ok that's famous but really great!)
Big Juss - Black Mamba Serums
Binary Star - Masters of the Universe
Blockhead - Music by Cavelight
Cannibal Ox - The Cold Vein
Company Flow - Funcrusher Plus
Dr Octagon - The Return of Dr. Octagon
El-P - Fantastic Damage
Glue - Seconds Away
Illogic - Write to Death
INI - Center of Attention
Invincible - Shapeshifters
J-Live - The Best Part
Juggaknots - Release
Jurassic5 - Quality Control (famous but amazing)
Look Daggers - Suffer in Style to listen the album : http://www.lastfm.fr/music/Look+Daggers/Suffer+In+Style
Micranots - Obelisk Movements
Nobody - Soulmates (it's a mix between the trip-hop and the undergroup hip-hop) to listen the CD : http://www.lastfm.fr/music/Nobody/Soulmates
Public Enemy - It Takes a Nation of Millions to Hold Us Back (famous again but really good)
Reflexion Eternal - Train of Thought
Sage Francis - A Healthy Distrust
Scienz of Life - Coming Forth By Day: The Book of the Dead
Viktor Vaughn - Vaudeville Villain
Wu-Tang Clan - The W
Yak Ballz - Homepiss EP - Yak Ballz to listen the CD : http://www.lastfm.fr/music/Yak+Ballz/HomePiss+EP