The following either have recieved way to much pub or have enjoyed way too much commercial success to be considered underground:
Talib Kweli, Mos Def, Gangstarr

The following have just recieved way too much pub to be considered underground:
Cormega, Tragedy Khadafi, edited to add Canibus

Any 12 year old suburban white kid with "Gansta Delusions" has heard of these cats. Underground my ass.