Hmm... some bands that most people may have not heard of... these aren't all underground so don't bother flaming, just bands the majority probably won't know or atleast I havent seen anyone talk about them. although maybe heard of some of there songs.
byzantine
candlebox
cinder
diecast
strata
e. town concrete
earshot
ether seeds
egypt central
fastball
taproot
fear factory
fenix tx
grade 8
gravity kills
blindside
hurt
killing joke
lustra
marcy playground
one side zero
orgy
poisonblack
adema
revelation theory
semisonic
seven mary 3
sick puppies
soil
black stone cherry
tantric
the smash-up
the union underground
thornley
zeromancer
----
plus if you want to know some death metal etc. bands because as swan said most consider it a underground genre (although some of these don't fit into the catagories I forgot to post them above as they are in a different section of my music )
destruction
tankard
kreator
sodom
ektomorf
sepultura
soulfly
chimaira
vader
nile
bohse
anata
acid bath
ill nino
cannibal corpse
mayhem
There are hundreds of others I can name but I think this post is big enough to give you a rather wide variety of different music