I'm going to +1 most of the stuff said. I haven't seen Mars Volta here yet, Anglagard , Gentle giant?
Older stuff:
Mahavishnu orchestra - Inner Mountain Flame and Birds of Fire are mandatory listening.
Yes - Close to the Edge is definetly a "must listen"
King crimson - Court of Crimson Kings, Elektrik/ConstruKtion of Light, stuff with Bruford
Jethro Tull - Aqualung, Thick as a Brick, Songs from the wood
Beatles (Sgt Peppers and Abbey Road)
Rush - 2112 and Moving Pictures
Yes - Close to the edge
Gentle Giant - In a glass house at least!
Return to Forever - romantic warrior and hymn of seven seas are some of the ones I introduce people to
UK (bill bruford, once again!)
newer prog waves:
Anglagard (Epilog and Hybris are mandatory listen! instrumental prog)
Animals as Leaders
Scale the Summit
Flower kings
IQ - dark matter
Isildur Bane - Mind.volume2 (this is insane!)
Liquid Tension Experiment volume 1 and 2 is required listen
OSI as recommended
Transatlantic - Bridge Across Forever and SMPTE
Spocks Beard - THoughts
Neal Morse - ?
Opeth - Heritage (simply stunning)
The Tangent - A Place in the Queue
towards the prog metal
Cynic - traced in air
Dream Theatre - Scenes from a Memory Part 2,
Ayreon (if you can stand the lyrics and the 80 heavy metal influence, has some good stuff in there. Try Human Equation or Universal Migrator)
Gojira \m/ new album is pretty cool!
Mars Volta - DeLoused in Comatorium and Bedlam in Goliath are amazing
Mastodon - crack the sky. This album ROCKED my 2011 year! (and 2012)
Meshugahh - Nothing, Obzen
Opeth - Ghost reveries, Watershed
Periphery
Symphony X
Planet X
Tesseract - concealing fate
Between the Buried and Me - Parallax is a good one to start with
Tool (lateralus and 10.000days are essential listen)