Does anyone know of any very powerfull moody classical music, like “O’Fortuna”, and “Duel of the Fates” from Star Wars? These are the only ones I know of.
“Mars the Bringer of War” by Gustav Holst (from “The Planets,” which inspired much of the “Star Wars” soundtrack")
“Night on Bald Mountain” by Modest Moussourgsky
“The Scythian Suite” by Sergei Prokofiev
Dude, could you maybe ask a broader question? 
By moody, I’ll assume you mean either dark or full of tension, if not both. “Duel of the Fates” is just one of John Williams’ really good jobs in the Star Wars series. Why not the Imperial March, or the Emperor’s Theme?
“O Fortuna” is merely the first (and last) movement of Carmina Burana: canciones sacrae et profanae. Carl Orff did a great job there, and you might want to look at the rest of the piece. I recommend the second movement – can’t remember the name, but the first verse is “Fortune plango vulnera/ stilantibus ocelis/ quod su a mi chi munera/ subtrahit rebelis/ verum est quod legitur/ fronte capilata/ sed plerum que sequitur/ ocasio calvata.” you’ll hear an awful lot of what inspired Williams in Episode 1’s “Duel of the Fates.” Also, “Circa Mea Pectora” (can’t remember the movement number) is another good choice.
While we’re on the subject, how about the first movement of Beethoven’s 5th Symphony (lots of power and tension)? Or Mozart’s Adagio and Fugue for organ (the quintessential creepy organ music)? Or Bach’s Adagio For Strings (extremely tense and sad)?
If you want soundtrack-level music on par with Williams, try Basil Poledouris. Two of his orchestrations (The Hunt for Red October and Conan the Barbarian) will provide lots of terrific fodder. Conan has a great and very listenable soundtrack and is one of my favorites. I haven’t listened to the music from the sequel, but it’s nowhere near as good IIRC.
I’m not a classical specialist, and I know I’m leaving out a lot of things I should mention. But there’s a tremendous amount of work just like what you;re looking for, and these would be great starts.
Addendum: Upon preview, I see that astorian has added some great tracks that I was too thick-headed to remember. Another one you might like a whole bunch is In the Hall of the Mountain King by some European guy whose name escapes me.
You might also try Symphony of Sorrowful Songs by Henri Gorecki, and the Gladiator soundtrack. There’s also Barber’s Adigio for Strings. Its not the uplifting type of music like O Fortuna, but a deeply moving piece, none the less.
Thank you, Tuckerfan, for correcting me composer-wise. Barber was the string adagio boy.
I am thinking powerful, like Mars, that is a good one.
You mean catchy accessible stuff you’ve heard somewhere before. 
How about - “Thus spake Zarathustra” by Richard Strauss. The opening of Dvorak’s “Slavonic Dances”. Satie’s “Gymnopidies” and “Gnossiennes”. Elgar’s “Nimrod” from the “Enigma Variations”. Vaughan Williams’ “Greensleeves Fantasia” and “Lark Ascending”. And too much Mozart, Vivaldi, Handel and Bach to mention
Edvard Grieg, and I think In the Hall of the Mountain King is a great suggestion.
Also, the section entitled “Montagues And Capulets” from Prokofiev’s Romeo and Juliet: it opens slowly with a couple of deafening crescendos and then goes into one of the coolests riffs in all of classical music.
Sibelius. Finlandia and En Saga, for definite. Luonnatar if you don’t mind sopranoes, and Tapiola if you want brooding rather than stormy. And the finale of the Fifth Symphony - “Thor swinging his hammer”.
Fantasia on a Theme by Thomas Tallis (Ralph Vaughan Williams)
The fourth movement of Dvorak’s New World Symphony, the “Liebestod” from Tristan und Isolde, the Funeral March from Siegfried(it’s used in the opening of Excalibur, the Gods’ Entrance into Valhalla from Das Rheingold, the Magic Fire theme from Die Walkure,
the “Rex Tremendus” from Mozart’s Requiem.
A few orchestral movie soundtracks off the beaten path, but personal favorites, and I think right up Muad’Dib’s alley:
Hellbound: Hellraiser II – by Christopher Young
The Final Conflict – by Jerry Goldsmith
Where Eagles Dare – by Ron Goodwin
And, sort of spanning the gap between movie soundtracks and “real” classical music, would be Prokofiev’s score for Alexander Nevsky; particularly the Teutonic Knights’ theme, which was glommed by John Milius for the above-mentioned Conan the Barbarian soundtrack
Also, while the Funeral March gobear cites is indeed that of the character Siegfried, I believe it technically is found in Die Gotterdammerung, rather than Siegfried. There are a number of Wagner anthologies that contain the piece, and other suitably bombastic stuff, so there shouldn’t be too much confusion anyway.
Absolute best possible answer to this question:
4th movement of Beethoven’s 9th symphony (ode to joy)
Other good answers:
The Firebird Suite by Stravinksy
The 1812 Overture by Tchaikovsky
the 4th movement of Shostakovitch’s 5th Symphony
The Verdi Requiem
The final two pieces from Moussorsgy’s Pictures at an Exhibition, as orchestrated by Ravel (The hut of Baba Yaga and The Great Gate of Kiev)
The Second Movement of Beethoven’s Seventh Symphony.
Beethoven’s Symphony No. 5. You’re probably familiar with the first movement, which probably contains the most recognizable four-note motif in the world. But you really must listen to the entire four-movement symphony to get the full experience - the omninous first movement, resignation in the second, the dark and brooding third, and finally the triumphant fourth movement. You’ll hear echoes of that opening motif in all four movements, and it says something different each time. Sit back and turn the volume to 11.
I’ll second the Verdi Requiem, especially the Dies Irae.
And I’ll repeat what I said in the thread I need oppressive choral music.
The ballet “Daphnis and Chloe”, music by Ravel. It contains the “Danse guerriere”, the war dance of the pirates, at the beginning of the second part. Very effective. There is a bacchanal (“Danse generale”) at the end of the ballet, an incredible piece of music.
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The ballet music was written to include a wordless chorus (they sing “ah”, “ooh”, and sometimes hum). However, it is sometimes recorded without a chorus. Make sure you get a recording with a chorus. There are many good recordings of the complete ballet. One nice LP performance reissued on CD is “Ravel: Daphnis et Chloe Complete / Munch, Boston Symphony 1993-08-10 / RCA Victor Living Stereo - 61846”.
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There are also many recordings of two suites which contain excerpts from the ballet. Suite #1 ends with the “Danse guerriere” and suite #2 ends with the bacchanal. You may want to check these recordings out. And again: get a recording with a chorus.
Two more.
Debussy’s second nocturne for orchestra, Fetes (“Festivals”).
The third section of Debussy’s La Mer.
The prologue to “Mefistofele” by Arrigo Boito.
Powerfull music that evokes the very Throne Room of Heaven.
Boito begins his retelling of the Faust legend in a manner very like the Old Testament book of “Job”.
Well from the all of three seconds of the sound track of “The Fellowship of the Ring”, I think it might fit the bill exactly (exept for the Enya bits:)).
The last movement, “The Storm,” from Gustav Mahler’s first symphony. A real window-rattler.