Opening acts you liked

Max Webster opening for Rush

I remember loving Spock's Beard when they opened for DT on the Metropolis 2000 tour. I didn't know them before but they really rocked the crowd - it's rare that you see such a confident support band.

Iggy Pop opening for Red Hot Chilli Peppers (2001 I believe)
Pain Of Salvation opening for Dream Theater (Six Degrees Tour)
Slipknot opening for Metallica (2004) 
 

I was really impressed by a band called mindfeed that opened for symphony x for the twilight in olympus tour I think. I though they were really heavy but it's probably because it was my first metal gig ever (actually my first gig of an kind)

I would probably say Devin Townsend opening for Children of Bodom, and DT opening for Maiden in 2010.
 
I could mention Megadeth/Slayer/Anthrax opening for Metallica, and bands such as SymX, Overkill, DT, etc. opening for Megadeth at Gigantour, but I think that's cheating.

Santana opening for Dave Matthews Band, circa 1997

DT for Maiden (it was just a fun concert!)
Macca for Macca :P
Zao for Demon Hunter (wish it would have been the other way around, though)

Lamb of God opening for Metallica
PoS for DT
Dead Letter Circus for Muse
Kate Miller-Heidke for Ben Folds
Augie March opening for Crowded House
Evermore opening for Split Enz
 
I think that's about it... :P

Dream Theater opening for Iron Maiden
Mastodon opening for Alice In Chains
Zappa Plays Zappa opening for Dream Theater
Skindred opening for Disturbed
Today I Caught the Plague opening for The Devin Townsend Project
 

I saw Alejando Escavedo and Hacienda open for Los Lonely Boys.

I never heard of Escavedo before, but he apparently started in a punk band called the Nuns in the late '70s. He played acoustic guitar and was accompanied by another acoustic guitar and a female violin player. They were I credible. She was literally shredding her bow. (I will leave that to Violin Theaters opinion of that is actually a good thing). Anyway, I saw them in a small theater. At one point, the totally unplugged and stepped in front of the microphones and played at the edge of the stage. I was lucky enough to have 2nd row seats. They were literally playing to us. It was an incredible show.

Easily my favorite was Spock's Beard for DT on Metropolis 2000 tour.  I had just heard of SB a few months prior, but had no idea how great they were.  They played The Doorway and The Light among others and I was hooked.  Neal almost stole the show before DT had a chance to start. 
 
 
 

Tenacious D opening for Foo Fighters last week in Sydney :)

Dark Suns (opening for Pain of Salvation in 2005)

Circa 1982, I saw the Police at Shea Stadium. We stayed in the parking lot for a little band named R.E.M.

Isis opening for Tool
Meshuggah opening for Tool
Big Gigantic opening for STS9

Riverside opening for Dream Theater (2007)
Stick Men opening for Porcupine Tree (2009)
Pain of Salvation opening for Opeth (2011)

KyleTheAngry


Kate Miller-Heidke for Ben Folds

My goodness, yes! Her studio recordings are alright; but live, she was absolutely astounding! Between her breaking out into opera, and Keir's phenomenal guitar playing, it's up towards the top of my list too. 
 
I probably would have enjoyed PoS more if it weren't for the 98.3% bass guitar mix.

I've been lucky enough to see a ton of good, energetic young bands as openers, as well as more traditional "support acts" (name bands that either want a larger audience or just can't justify the size of the venue as a headliner).  Some of these are Fastway, Queensryche, Cinderella, Twisted Sister, Def Leppard and Bon Jovi in the former, and Motorhead, King Crimson, Blues Traveler, Ted Nugent, and Dio (yes, Dio) in the latter. 
 
But the best opening act I ever saw and coincidentially the first concert I ever saw) was Iron Maiden opening for Judas Priest on the Number of the Beast tour.  Wasn't their first exposure to the US (they toured with Priest on the Killers tour), but it was Bruce's first run through America, and I am not joking or exagerating when I say he set the standard that all other front men have been trying to live up to ever since.  What an amazing show.  And even though I will admit that some of it might have been the newness of seeing a real live rock concert, but I've heard tapes of that tour since then and it was every bit as good as I remember it. 

Andy McKee (for Dream Theater, 2011)
 
Holy smokes.  Never heard of the guy, had no idea there was even an opener that night.  This guy was amazing, and I'm just getting into him now.  Contrast that with the absolute worst opener I've ever seen, which was also for Dream Theater in 2011:
Trivium
 
Other good openers I've seen:
Queensryche (for Chickenfoot)
Collective Soul (for Van Halen)
 
Interesting/unique openers:
Polyphonic Spree (for Brian Wilson's "Smile")
 
Bad Openers:
Skid Row (for Van Halen, couldn't bring it live)

BTBAM and Opeth then Zappa plays Zappa, are hard to top.
 
As a Mars Volta fan I've seen them open for APC and Soundgarden, and boy do they suck live. 

3 opening for Opeth. Knew only that they played at Prog Nation but never checked any music. While waiting around outside on a patio with beers waiting for the first opening to finish we heard 3 was going on.
On Portnoy's positive words about them I and my best friend wanted to check em out so we all went in. These fuckers blew our minds with their live show. I will honestly say that as a fan of Opeth and seeing them for the first time 3 stole the show for me. Actually for all of us in the group. Not to say Opeth sucked cause we loved it, but it was just such an amazing surprise to be glued to this opening act. 
Needles to say, I am now a fan of 3, yet another good recommendation by MP, so go out and get The Ghost You Gave to Me!
hehe, that rhymed nicely... maybe?  HA! hehe