I don't know too many people who are into it or rap in general, but I was thinking mabye people here might be. here's some genuine rap reccomendations that I listen to (and I'm more of a rock/alt person). half of this isn't really underground at all but counts in comparison to this mainstream t-pain stuff.

AZ
Talib Kweli
Afru-Ra
Juice
Mos Def
Masta Ace
Access Immortal
Immortal Technique
Non Phixion
Sabac
Cormega
Zion I
Asheru
Common
7l & Esoteric
Big L
Choruss (russian/saarland)
Celph Titled
Do or Die
Eyedea (a little out there)
Jedi Mind Tricks
Vinnie Paz
T-lo
Tragedy Khadafi
Scarface (Brad Jordan, lots of compilation albums)

That's my main stuff, but there's always like nwa and jay-z and stuff. even some dipset is pretty good.

Murs & Nas

i like aesop rock (so fuck you)
Cage
Dr. octagon

I like a lot of the same stuff as you, check out:
showbiz & AG
naughty by nature
Slick Rick
D.I.T.C / Children of the corn
Onyx..
Del Funky Homosapien
mac dre

pharoahe monch

I don't really make a distinction between underground and mainstream rap, but here's just some good albums you might have not heard... old and new, some known, some obscure:

Get Low Playaz - Straight out tha Labb
Black Milk - Popular Demand
Wreckless Klan - Blowin Up Da Scene
D.I.T.C. - s/t
Dayton Family - F.B.I.
Triple Darkness - Comin' Up From the Darkness
Oddisee - Foot in the Door
Phat Kat - Carte Blanche
Crooked Lettaz - Grey Skies
Scientifik - Criminal
Uncut Raw - First Toke
J.D. Walker - The Career Criminal
Jake the Flake and the Flint Thugs - s/t
San Quinn and Messy Marv - Explosive Mode
Prophet Posse - Body Parts
Slum Village - s/t
Cocoa Brovaz - Rude Awakening
Lil Maniac - Cold and Heartless
A.W.O.L. - What It Be Like


Runaway Slave, one of the best hip hop LPs ever.

just some 2008 stuff

akrobatik - absolute value
astonish - from now until forever
az - undeniable
black spade - to serve with love
buckshot & 9th wonder - the formula
chaundon - carnage
danny brown - hot soup
elzhi - euro pass
fat ray & black milk - the set up
guilty simpson - ode to the ghetto
invincible - shapeshifters
j dilla - pay jay
kam moye - self centered ep
kev brown & lmno - selective hearing
life cycle - city of rust
lone catalysts - square binizz
nicolay & kay - time line
now on - dont call it a mixtape
oddisee - good tree ep
rashid hadee - a change gon' come
tanya morgan - tanya morgan is a rap group
torae - daily conversation
verbal kent - fist shaking

there's an interesting variety here

the corner of spec & 9th - spectac and 9th wonder
9th gate - access immortal and 9th wonder

these fit in with that list.

Mr.Lif
Tame One
Necro
Ill bill
Apathy
Copywrite
jakki da motamouth
Slow suicide Stimulus
Dusted Dons
Fat Lip
The Pharcyde

Oh I love Rap music.

Check out UGK.

Bun B
Pimp C

Heavy D
Easy E