Bands/artists you never got to see live in their heyday, but would've liked to.

B.B. King

Albert King

Stevie Ray Vaughan 

@tvad, say WHAT? The Minutemen >>> Bonnie Raitt? That's more than my mind can take in!

I saw a relatively unknown Van Halen open for Black Sabbath; a completely unknown U2 play for a small crowd in a club; an up-and-coming Stevie Ray in a small club; I  sat in on a Bonnie Raitt soundcheck once which was a hoot; and so on. But I ain't never seen The Minutemen open for Bonnie Raitt!

(My wife saw the Ramones open for Cheap Trick, another combination that is hard to fathom...)

@jerryg123 I saw Amy Winehouse in concert and was impressed enough to buy her record.

Nirvana 

Dire Straits

Dead Kennedys

Black Flag (Ron Reyes era)

Discharge

Blitz

Doom

X-Ray Spex

Generation X

Operation Ivy

The Undertones

The Purple Hearts

Secret Affair

Toots & The Maytals