Well, by my own personal definiton of epic, here are a few songs that I think might pass for epic (note: I don't necessarily like all of these, they are just examples of 'epic' songs):
Nightwish - Ghost Love Score
Dimmu Borgir - Progenies Of The Great Apocalypse
Opeth - A Fair Judgement, The Drapery Falls, When
Portishead - Roads (in an understated way)
Porcupine Tree - The Start Of Something Beautiful, Strip The Soul
Yeong Wook Jo - The Last Waltz
Paradise Lost - It's Too Late
Borknagar - Collossus, Revolt
Arcturus - Ad Astra
Anathema - Flying, Temporary Peace, One Last Goodbye, A Dying Wish
Tool - The Grudge
Antony And The Johnsons - Hope There's Someone
Bjork - Pagan Poetry
Twinzero - Monolith
Dead Can Dance - Summoning Of The Muse
Blut Aus Nord - Procession Of The Dead Clowns
Dream Theater - This Dying Soul
Emperor - The Eruption
My Dying Bride - A Doomed Lover, The Wreckage Of My Flesh, The Deepest Of All Hearts
Elbow - Switching Off
Muse - Citizen Erased, Butterflies And Hurricanes
Pearl Jam - Black
Pelican - Angel Tears
Radiohead - Paranoid Android
Rufus Wainright - I Don't Know What It IS
Deftones - Minerva, Change In The House Of Flies, PAssenger
Sigur Ros - Hoppipolla
Explosions In The Sky - The Earth Is Not A Cold Dead Place (album)
Cult Of Luna - Leave Me Here
Tori Amos - Can't See New York, Precious Things, Northern Lad, Icicle
Winds - Visions Of Perfection
And anything by Hope Of the States, especially Black Dollar Bills and The Black Amnesias