This will be biased based upon some recent listening, but the first I will mention is Gina Sicilia: Neo-soul singer, very large-voiced, tough alto singing, but she can also perform quite beautifully on more mellow tunes. One thing that I love is that while the girl can sing in many different styles, and hit some higher notes here and there, she does not try to over-extend herself outside of her range. Sometimes, it can be very difficult to find an alto that doesn't suffer from soprano envy. This is not the case here. The album Can't Control Myself is brilliant.
I've also discovered Dana Fuchs recently, who also has a somewhat rough, tough voice, and while she doesn't quite have the range of Sicilia, she is a talented singer. The closest comparison I can think of is Janis Joplin before her voice was wrecked. Hell, she got her start performing by being part of a two woman play about Joplin's life and had to sing twenty Joplin songs a night. (Yes, twenty! I am pretty sure that Janis couldn't sing twenty Joplin songs a night...ninja) Her studio album is mediocre, unfortunately, since a scattershot approach is taken in regards to musical styles, but her live stuff is great. Check YouTube for some songs.
Another is Eden Brent, but be warned that she has a very love-it-or-hate-it voice; sort of raw, over-cooked, smoky, southern vocals, quite distinctive. For a point of reference, try to picture an extremely southern Billie Holiday while she still had some strength in her voice. I especially love the backing instrumentation on her albums which consist of a lot of acoustic blues/boogie woogie/New Orleans type material. Ain't Got No Troubles is great.