Ooh, lots of prog rockers here
Here are some reccomendations-
Shadow Gallery - Anything by them, I like Carved in Stone the most, Legacy and Tyrrany are both excellent. Occasionally they remind me of Dream Theater, but they are less showy, and have much more acoustic material. I also like their singer a good bit more, and I find their music would probably be more accessible then most prog rock (More melodious). They make much more heavy use of synthesized piano then most groups, and they use flutes too.
Symphony X - Take Dream Theater, give them a darker, harsher sound (Symphony X has more of a crunch to it), and more synth is used. My personal favorite songs by them are on the very hard to find Divine Wings of Tragedy, with the feature song based on Holst's Mars, and is kind of long at 22 minutes or so
. The other song that stands out for me is Candleight Fantasia, which is more like a ballad, and it has a dark, eerie tone to it. The next best CD IMO is V, which is much easier to find, and has a harder edge to it, and they use a lot more dissonance. Choral arrangements are the norm too, they make fairly heavy use of multiple singers and harmonies. Another favorite song of mine is Egypt (On V). I don't like them as much as DT because they can have a more dissonant sound at times, so this might be something that you like about them.
Liquid Tension Experiment - 2 CDs, both incredible. They are made of 3/4 of DT and it shows, just substitute Tony Levin for Myung, I personally like Levin more. A lot of their music is improv, and some is written beforehand. The first CD was produced inj a week, the second in 2 weeks or so. The music could simply be described as "insane", they just play their hearts out and it shows. My favorite songs on the first one are Paradigm Shift and Kindred Spirits, and on the second one I like Biaxident and When the Water Breaks, some of the imrovs are really good (Three minute warning is wonderful, and the second track on the second disc is extremely good IMO).
Bozzio Levin Stevens - Tony Levin is here again, yay for him. This group can be described as a more mature sounding group like LTE, but they don't have a keyboardist. My favorite song on Black Light Syndrome is The Sun Road, but others are excellent too. They have a more laid back, relaxed sound then LTE and sound like thay have had more time playing music together. They also make use of a more spanish style guitar, which is quite nifty.
Gordian Knot - Insturmental, more like Bozzio Levin Stevens, but has a jazzier sound to it. I don't have any of their CDs but I like my mp3s of them
.
Arena - Prog metal, has a bit of an 80s kick to them, and make use of a fair bit of synth. Someone here calls them his favoirte band (I think it might be Braver), and I strongly reccomend them.
If you like Shadow Gallery and want to try something a bit heavier, try Royal Hunt.
If you want to hear something that is slightly like DT but has a more modern, poppish sound try Dali's Delimma. And I associate the song Ashen Days with my car, when I hear it all I can think of is how it sounds when played in my car.
Tempest - I don't have any of their CDs but I wouldn't mind owning one, they are more folky and have a laid back sound.
If you like prog rock and prog metal, I would also reccomend trying some Therion and some Blind Guardian, you may like one of them. Therion is dark operatic metal, Deggial could be considered prog too (Via Nocturne is an extremely good song) and Blind Guardian has a new single out (And Then There Was Silence) which I strongly reccomend, they play power metal, and you could also consider it progressive since it can change a lot. Blind Guardian has a very triumphant sound,
You seem to have extremely eclectic musical tastes, and because of that I can practically garuntee that you will be quite amazed once you start exploring this genre.
If you want more reccomendations, I can give you more, but you will be swamped
.
Edit: Just started reading Lord Chimp's reviews, and I agree with him very much on Symphony X
.
I have mentioned these guys several times here as reccomendations, and noone has bit. I HIGHLY reccomend you take the plunge with them. DWOT is such an extremely good disc, hell V is so good that after 2 songs a friend of mine called them his second favorite group. Some people say they are overrated, those that do need to listen to them again or something!
