Interesting thread, this!
I'm a big fan of both old and new progressive rock and metal. I agree with the theory that "progressive" means to progress, to push boundaries and move forward. Unfortunately, too many prog-metal bands think being "prog" means to try and compete with DREAM THEATER in the widdly solos stakes....
A few of you have mentioned RUSH, which is great. I'm an absolute fanatic of theirs, although the problem is whether you become a fan or not really depends on which albums/songs you hear first. In otherwords, like The GATHERING they have evolved constantly.
A potential fan may hear one of their albums and then dismiss them without realising the rest of their stuff is so different. Maybe the best way to start would be to get their double-cd Best-of album "Chronicles", but the RUSH albums I would recommend are:
From their heavy metal era:
2112 (1976)
Hemispheres (1978)
For the prog-rock/metal crossover:
Permanent Waves (1980)
Moving Pictures (1981)
For those who want to try their more synth-rock period:
Signals (1982)
Grace Under Pressure (1984)
And for their combination of hard-rock and prog:
Counterparts (1993)
Test For Echo (1996)
From the ZEPPELIN-on-crack heavy metal of their 1974 debut, to the synth-rock of "Signals" in 1982, to the alternative hard rock of "Test For Echo", they have never done the same album twice. A "progressive" band in the true sense.
As for today's scene, I think the best modern prog-rock and prog-metal bands out there now are:
SYMPHONY X
EVERGREY
DEADSOUL TRIBE
PORCUPINE TREE