Here's the list thus far:

Rachmaninov's Symphony No. 1 - the four-note "danger" motif
Mahler's Symphony No. 8/Schindler's List - the main theme, I guess
Shostakovich's Symphony No. 11 - Danilov's Confession
Prokofiev's Alexander Nevsky - the "Field of the Dead" melody makes an appearance in the opening track

and here's a new one I just found out:

Philip Glass's Timpani Concerto: Second Movement (the last few minutes)

There's this recurring harmonic progression throughout the film that provides a dreary atmosphere, sometimes accompanying the four-note motif. It's straight out of this concerto. Pretty nifty!