Black Sabbath has a few songs, esp. from the Dio era, that could work for D&D. Neon Knights (pushing it, really), Heaven & Hell, Lady Evil, Mob Rules, etc. Even some of the Sabbath stuff after Dio (the Tyr album comes to mind).
Led Zeppelin has been covered well enough, I think.
Manowar & Armored Saint, as mentioned earlier, are great.
A couple of Dethklok/Metalocalypse songs work, too, though in a lighthearted vein: Awaken, Lost Vikings, Thunderhorse.
Iron Maiden has been mentioned: Flash of the Blade (from the Powerslave album) could work, and Number of the Beast could work for a sinister cult (Orcus, Mephistopheles, etc.).
Bruce Dickinson's solo work works as well: The Magician, The Tower, The Alchemist, King in Crimson, Tears of the Dragon.
Old Judas Priest stuff can work, too: Tyrant, for example.
The Conjuring by Megadeth could work for a sinister cult theme.
Motorhead has a few songs that could work, too: Deaf Forever and I Am the Sword are a few I know off the bat.