What albums would say are "essential progressive metal albums"
 

I'd say Octavarium by Dream Theatre for the sheer technicality and intricate song structure, although I do find it a little cheesy now, myself.

Lateralus by Tool is one of my favourites for its lyrics and pummeling, yet intricate riffs and time signatures.

I think from my experience, Mattias Eklundh's album "Freak Guitar" is the most virtuosic album in this genre, he's certainly one of the best metal guitarists around today.

Not really a fan of metal in general, but a great prog/post metal record you may enjoy is

Russian Circles - Empros

http://1.bp.blogspot.com/-BYEFQ43JMA...les-Empros.jpg

Released last year, it features some of the best instrumental music I've ever heard.

Dream Theater's Six Degrees of Inner Turbulence, most any album by Tool, but I'd say that Aenima is their definitive piece, and Opeth's Deliverance seem to be prime staples of the progressive metal genre. Porcupine Tree gets a lot of props for what they're doing, but they rarely bring in metal influence in their songs, and seem more like a somewhat heavier Pink Floyd most of the time.

The Norwegian band Green Carnation has put out some pretty impressive material like Journey to the End of the Night (2000) and Light of Day, Day of Darkness (2001), the latter of which was an hour long ode.

Devin Townsend's debut solo album Ocean Machine - Biomech (1997) is also one of the best things he's ever produced which is a pretty impressive feat given the man's discography.

Nevermore's 1999 album Dreaming Neon Black is probably their most underrated and continuous their ability to combine elements of power and thrash to a more progressive rock edge.

Then of course you've got bands like Sigh who are just all over the map with their genres but their 2001 album Imaginary Sonicscape felt like something that would have existed in the 1970's in Bizzaro World. Their heavy use of old Moog snyths (I think they actually credited Moog as a member of the band in the liner notes) really help make it feel like an old Canterbury scene album from the '70's, which really helps move the genre away from its usual pigeonholing of just being heavy metal songs that don't feature growled vocals, but go on for longer than most heavy metal songs.

I'd agree with everyone's suggestions so far. Very well said everyone. Salami, don't forget Freak Guitar: The Road Less Traveled [there's two albums].

Dream Theater have great albums, but what introduced me to the genre as a whole was their instrumental project without James LaBrie, Liquid Tension Experiment.

To me, the self-titled Liquid Tension Experiment and the follow-up album are two of the most important prog-metal albums.

Actually, it was you who introduced me to his music, I've only listened to the first one but I think I definitely ought to give The Road Less Travelled a proper listen.

I can't really say that Mattias fits in in this particular instance. He's amongst the Satrianis, Bucketheads, Anastassakis', Gilbert's, etc. He puts on a damn good show, though.

He has a full band with vocals as well, but it's cheesy metal. Just for fun stuff. Makes you laugh. They're called Freak Kitchen. Have a great handful of above average tunes and amazing guitar solos, but no real solid album.

Some that RUSH immediately to my mind :
Meshuggah - Destroy Erase Improve
Fates Warning - Awaken the guardian
Queensrÿche - Rage for order
Dream Theater - Images and words
Watchtower - Control and resistance
Voivod - Nothingface
Crimson Glory - Transcendance
Pain Of Salvation - The perfect element, pt I
Psychotic Waltz - Into the everflow

No particular order
 

These are the ones I recall right now:

Cynic - Focus
Atheist - Elements
Tool - Lateralus/10,00 Days
System of a Down - Mesmerize/Hypnotize
Extol - Synergy/Undeceived
Believer - Dimensions
Galactic Cowboys - Space in Your Face/At the End of the Day
Tourniquet - Microscopic View of a Telescopic Realm/Pathogenic Ocular Dissonance