Technetium said:
Andromeda ("II=I" is one of the best albums of 2003)
Angra ("Fireworks" and "Rebirth" have mindblowing guitar playing and some great vocals to boot)
Arena (get "The Visitor", "Immortal?", and "Contagion")
Ayreon ("Actual Fantasy" and "Into the Electric Castle" are the two best)
Eternity X ("The Edge" and "Mind Games" in particular are amazing)
Flower Kings (I only have "Unfold the Future" but it is a great mix of prog rock and jazz fusion, with the king of guitar, ruler of all, Roine Stolt)
Iced Earth (Get older stuff with Matt Barlow before the new album, it's way better. Especially get "Something Wicked This Way Comes" and "Burnt Offerings")
Opeth (start with "Damnation", then wean yourself onto the heavier vocals with "My Arms, Your Hearse" and "Blackwater Park")
Pink Floyd (You forgot to list them in what you already know and like :-D )
Porcupine Tree ("In Absentia" is pretty good, "Signify" is much better, the others are out of print)
Queensryche (You should already know these guys)
Transatlantic (if you like long songs, I do and these are my two favorite albums of all time)
Shadow Gallery (start with "Carved In Stone" or the self-titled, if you can find it)
Yes.
YES.
Finally, someone else realizes the unsurpassable godliness of Roine Stolt and The Flower Kings.
You're my new best friend.
You need to get their other albums, since all of them are stunning. Also, check Tomas Bodin's solo side-project, as well as Karmakanic and The Tangent, two excellent bands that feature FK members, and are (somewhat) along the likes of the FK.
To the topic creator: if you're looking for something heavier but still proggy, check out Edge Of Sanity's album "Crimson II". Yeah, the regulars here will probably roll their eyes at my incessent pimping of C2, but it's really fucking good.
If you're looking for some prog at the rock end of the spectrum, you can't go wrong with Spock's Beard's amazing album "Snow".
If you're feeling a bit adventurous, try Bal-Sagoth, which is an amalgam of power metal, black metal, killer symphonic keyboard orchestration, pompous spoken-word narrations, and much more. One of my favorite bands.
Time Requiem is a great example of quality neo-classical metal. Kind of like Symphony X, but wankier.
Pain Of Salvation is a necessity. Brilliant stuff. Try either "The Perfect Element Pt. 1" or "Entropia".