Can’t remember if they were in the movie…but Eddie and the Cruisers/John Cafferty and the Beaver Brown Band
Chris Cornell is the unnamed guy who says something to Matt Dillon after Dillon blows the windows out on Bridget Fonda’s car installing a new stereo in it.
He actually doesn’t say anything, but disappears when the Windows shatter.
Tad Doyle of Tad was the guy who answered Bridgett Fonda’s wrong number.
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Big Bad Voodoo Daddy was the band in Swingers.
I haven’t read the entire thread, but I’m guessing this hasn’t been mentioned. I just watched “With Six you get Egroll” over the weekend so I’m nominating The Grassroots as the thread winner. How’d I do ?
Alice Cooper doing Because! Actually a pretty amazing version - especially considering the piece of crap the rest of the movie is!
OP explicitly DQ’s TV show examples.
In the movie Everything is Illuminaed, Gogol Bordello lead singer, Eugene Hutz, plays one of the main characters in the movie but does not perform. However, the rest of GB show up as a small local folk band in one scene. Here’s the clip, they’re the little band chasing the train: everything is illuminated - JonFen.avi - YouTube
They appeared as themselves and are credited as Big Bad Voodoo Daddy.
Scroll up to post #65.
Blues Traveler played in Kingpin, I don’t think they were billed as BT since they were wearing fake Amish beards.
Dunno if this works: John Cafferty and the Beaver Brown Band didn’t appear in the movie – they provided the *sound *of Eddie and the Cruisers, but the actors portrayed the band.
See post #1.
Not germane to the OP, but Wristcutters: A Love Story features a character named Eugene who is basically Hütz, and GB songs are portrayed as his, most prominently “Through the Roof ‘n’ Underground.” Tom Waits is also in it but I don’t remember if he plays anything. He’s not playing himself. Fun film.
Mostly because there are a ton of them. Buffy had many bands playing at The Bronze while exposition was happening at one of the tables. I think a lot of YA themed shows have done the same thing. I’m not going to ask for the thread to be shut down if TV examples are given but I figured movie examples will be much more rare. I commend the posters for coming up with some pretty damn obscure examples.
I’ll break my own stipulation and mention my favorite one from TV. The band Detective appeared on WKRP in Cincinnati as the fictional band Scum of the Earth. Lead singer Michael Des Barres and two actors played the band in the speaking parts but the full band was playing as Scum at the end of the episode.
Well, that’ll make you feel like a dumbass. I must have sublimally picked up on that by skimming the OP or something, I hadn’t thought of the band or the movie in years and yet that was what I came up with. Or maybe I’m just a dumbass.
Didn’t mean to make you feel like a dumbass – I’ve almost done that some thing plenty of times. Or maybe I’m a dumbass, too. 
Charmed comes immediately to mind.
In the movie Valley Girl, I’m pretty damned sure Josie Cotton and the Convertibles have some screen time.
What’s more, at the start of the film, Carrey’s character is shooting a music video…and the song is GnR’s “Welcome to the Jungle”:
In 1985’s Vision Quest, Madonna performs as an unnamed singer in a bar band (her IMDb credit is “Singer at Club”). The movie was released a couple years after her debut album, so she’d already made it big.