Stargazer (at 8:26) by Rainbow.
Not well known, but a hard rock classic.
I’ll put in a plug for the live version of Oingo Boingo’s “Change.” The 8:50 arrangement from Farewell is a lot tighter than the 16-minute album version.
Another vote for Sister Ray, and if you’re really in the mood, play Sweet Sister Ray before one of the live versions on the Bootleg Series for seventy minutes of listening pleasure 
“Won’t Get Fooled Again” by The Who.
(ARP synthesiser noise)
(Keith Moon drumming)
“YEAAAAAAAAOOOOOOOWWWWWWW”
That song is worth it just for that moment. To me, “Won’t Get Fooled Again” is the rock song over 8 minutes in length.
The live version of “Push It” on that Tool DVD/CD thing is pretty amazing too. Maynard James Keenan + tabla is a great combo.
Phish! Most of their songs usually end up being over 8 minutes live. Some of my favorites: “tweezer” (up to 40 minutes long), “Stash” (awesome), “You Enjoy Myself”. I don’t think I’ve ever not liked a Phish song, and they are verrry long.
Another vote for “Third Eye”. Probably the best Tool song ever.
And since he played it on Conan last week, “Kitty’s Back” by Bruce Springsteen. AWESOME.
The End, The Doors. Light My Fire is long too.
Layla is nice and long.
Kashmir by Zep
Abbey Road side two is a bunch of songs strung together and is magnificque!
Dark Star
Since you mentioned Thick as a Brick, most of the concept albums of the 1970s could be considered long songs even though many of them are broken up into individual songs. One prog-rockish album not mentioned here is “Foxtrot” by Genesis back when they didn’t let Phil sing (much). The second side is one song called Supper’s Ready and is very good.
In terms of newer bands, I would say that Phish and Stereolab have got the best long songs. The Fluffhead stuff from Phish’s Junta is magnificent. Stereolab puts a 7 min+ song on nearly every album, and they are often stellar. Not only Jenny Ondioline, but a newer track from the Dots and Loops album (I believe it is Refractions in the Plastic Pulse but I can never keep the track names straight) is also excellent. I think they deserve extra consideration because last week Mary Hansen, their second vocalist/guitarist/keyboardist was killed. That and they are one of my favorite bands ever.
Oh, I forgot to say, it’s not really rock but it holds fond memories for me as my dad and I used to play it to piss of my mom: Mike Oldfield, Tubular Bells side 1. Mostly for the kitsch value.
I really like Yes’ “Gates of Delirium”, which comes in at around 22 minutes. Had it not been for the limitations of LPs, which could hold about 23 minutes of music per side, I imagine Yes would have done even longer tracks.
2112
I was wondering when RUSH was going to get a mention. In addition to 2112, there’s also Cygnus X-1, which spans two separate albums (Book I on A Farewell to Kings and Book II on Hemispheres).
Mother Russia - Rennaisance How many of you remember them ?
Trip to the fair - Rennaisance, I prefer this but most go for M russia.
Autobahn -
In my time of dying by Led Zep.
Mother Russia - Rennaisance How many of you remember them ?
Trip to the fair - Rennaisance, I prefer this but most go for M russia.
Autobahn - Kraftwerk, I suppose that techno music mowadays probably makes this seem very dated but I still have a soft spot for it.
Fools gold - Stone Roses
Pick up the pieces - Average White Band, the version they played live on BBC’s late night ‘Old grey whistle test’ is pretty much a soul-funk jam session, great improvisation.
Can’t win - Richard Thompson, it seems odd to me that this very British artist is appreciated more in the US than here, the live version of this is worth seeking out.
Soul sacrifice/heads ahds feet - Santana, great track and the drum solo by Graham Lear is awesome
The Beatles’ “Revolution 9.”
Death Cab For Cutie - Stability (12 minutes)
Built to Spill - Untrustable Part 2 (About Someone Else) (9 minutes)
Jimmy Eat World - Goodbye Sky Harbor (16 Minutes)
I love my length emo.
I’m fond of “A Change of Seasons” by Dream Theater.
There’s a group “Death Cab for Cutie”? I love it! (Would you like to know where their name came from?)
RealityChuck:
Absolutely. Also “Dogs”. And “Interstellar Overdrive”. And “Hey You” from The Wall is also over 8, isn’t it?