Saint-Saens' Dance Macabre is one of my personal favorites. I'm kicking myself for this but I can't remember who compsed Tocatta in Fugue, I think Bach... anyhow that's the tune you hear played to phantom of the opera all the time or 20,000 leagues under the sea.
Rachmaninoff is... well, you either like him or you don't. I happen to very much. He can go from being sappy flowery to bruding or powerful and morose. His Vesper's is an interesting listen. It's entirely done in choir, kind if remniscent of gregorian chant (to me at least).
Mussorgsky, you definately might want to try him out. Good stuff.
I like Grieg, hall of the mountain king and some other ones you'd probably be familiar with. Nice, interesting, melodic, though maybe a bit sophomoric for some tastes.
Holst' The Planets deserves a listening to, you'd find some of the tunes familiar like Mars; Bringer of War. There are other really nice ones though that shouldn't be overlooked.
Beethoven very easily fits into your dark and moody request, one of my favorites.
Chopin.... meh. I like one of piano concerto's but I can't even remeber the name of it. Other than that, not my cup of tea.
Man there's so much more... I need to start dusting off my classical CD's.
EDIT: for horrible spelling, also another one I really enjoy from saint saens is Bachanelle. Sends shivers down my spine still