Stick it Out - Rush (yes, even Rush did Drop D at one point) - main riff is easy, the rest isn't as easy necessarily.
Anything by Tool. Schism is surprisingly easy once you understand the cadence of the main riff. It's good pick practice.
Testify - Rage Against the Machine - great introduction to Tim Commerford's style, with a lot of syncopation and some ghost noting in the main riff
Bullet for my Valentine and Avenged Sevenfold both play almost entirely in Drop D. They're also good pick practice.
Scars by X Japan is also in Drop D, in contrast to most of the band's other songs which are in Eb. The original line was played with a pick, but I personally like the sound I get playing it with my fingers. It's a good riff to practice bursts of speed.
Hope this helps.
EDIT: Also I forgot about Everlong by the Foo Fighters. That's a really simple one.