I am a low baritone, able to sing a double low G (G1) in modal voice and my top range is a A4. I have trouble singing anything beyond an A4, and lot of metal singers can go way beyond that. I know Peter Steele can sing really low, but who else can sing that low?
Roy Khan, guy from 69 Eyes, Mikael Akerfeldt, guy from Sabaton, Dan Swano to name a few.
On songs with just clean vocals, Mikael Stanne from Dark Tranquility has a pretty rich low voice.
If you want to try Roy Khan, stay away from his Conception albums.

I've always heard Matt Barlow's name when this sort of topic comes up.