I guess to be more correct I should say Eva Cassidy should have been a superstar while she was alive.
She did have an album go to #1 in 2001.
Eva Cassidy has had three #1 albums in the U.K. and one #1 single (her posthumous duet with Katie Melua). I don’t think Songbird was a #1 album in the U.S., even though some websites claim that it was. It was a gold record but not a platinum one in the U.S. Can anyone tell us the highest chart position of Songbird in the U.S.? Please don’t just quote the Wikipedia entry or something, because that’s what I’m questioning. Does anyone have a book of all U.S. #1’s or some such? Also, she has never had any #1 singles in the U.S. In any case, I would say that Eva Cassidy is still not a mega-superstar in the U.S., so she does belong in this thread.
I saw a video which included a picture of Billboard with Songbird at #1. Maybe it was a different chart than the main album chart.
I dunno what happened in new york but they were downright huge in the bay area in the 90s. Headlined festivals, constant radio play, etc. Maybe not mega-superstars, but definitely superstars. Of course I can’t stand them so I can understand the non-mega-ness 
Another vote here for Mika. And yes, he is very heavily influenced by Freddy Mercury (not so much Queen IMO, if that makes sense). One of the most underrated albums of the 2000s.
My contributions:
Modest Mouse - I know I am a broken record here, but I am going to keep pimping this band until they are recognized as what they were: the best band alive from 1995-2005. Sure they are a fairly big act now with some radio play and a decent following, but this band is a first ballot hall of famer in my book and I still get way too many "Who?"s when I bring them up in casual conversations. They rocked, wrote amazing songs, and have the best lyrics in rock music history.
Eels - Had a modest hit with Novocain for the Soul, but he has a whole catalog of excellent, catchy, radio-friendly songs, including my favorite: Last Stop This Town
Islands - Everyone I play their good songs to (Volcanoes, Jogging Gorgeous Summer, Vertigo - cant find any good links sorry) loves them.
While XTC had a number of minor hits, they should be much better known than they actually are.
If we are talking acts that didn’t make in the States… Irish boy band are huge everywhere in the English speaking world except North America. Those Irish voices (swoon)… ex Westlifer Brian McFadden who has gone solo & now lives in Australia is also worth checking out.
NZ band Split Enz should have been huge (although Neil Finn went on to fame with Crowded House)
I’m going to check out a few of the acts named in this thread - I’m always on the lookout for something different.
Good onya mate 
My buddy filmed that first video. Geoff’s in the US this month, so, as they say, check local guides, and he’s far, far better now than he was then.
Oops my post should read Irish boy band Westlife are huge …
Me too! (Warning, WMA)
On a serious note, I always thought that Fates Warning should have been much bigger. They are a bit prog but have a bunch of stuff that is great and radio friendly.
Slee
That was cool, Slee (what does WMA mean though?)
& whoever recommended Vienna Teng - thank you. I have checked out a couple of her tracks & they are lovely. Exactly the sort of music I like to listen to. 
It’s the file type (.wma, Windows Media Audio) of the media link. Some people like to be warned before clicking on a link that prompts them to download a file. You’ll most often see this warning with links to PDFs, which can do screwy things in some browsers.
OK thank you!
I’m so out of touch with the modern world over here in Okinawa that I don’t know what’s mainstream and what isn’t anymore, so Andrew Bird may be a superstar for all I know. But he’s awesome. Fake Palindromesis one of my favorite songs.
Thanks for starting this thread, OP. I’m especially loving Mika–extremely catchy.
Daylight Basement is now defunct, but their one album was pure pop perfection.
ETA: They had one video too.
I’m also a bit surprised that Lux Interna isn’t getting played more. “Into Nothing” is brilliant, though admittedly their other tracks aren’t as good.
I would be happier than a pig in shit with that, as well.
I dunno; maybe Dave Dee, Dozy, Beaky, Mick & Tich ?
My vote goes to Tim and Neil Finn. Also Neil’s band Crowded House too!
Linky broke. Me fix: http://www.youtube.com/watch?v=kD02PU28NNw
Emitt Rhodes should have been a superstar 40 years ago; he was called the “One-Man Beatles”, with the voice of Paul McCartney and the singlehanded studio genius of Todd Rundgren; he served as the singer in Merry Go Round, cranking out kick-ass early-Beatles-esque jams, and then released four stellar solo albums that rivaled anything put out by any of the members of the Beatles in their solo careers. The guy had it all, insane instrumental talent and studio knowledge, clever lyrics, very good looks - and what happened to him? Like the Colossus of Rhodes and the nation of Rhodesia, Emitt Rhodes vanished into the ether. (You’ve got to say that line in a British accent, to get the full poetry of it.) It’s a fucking tragedy. Thank God he’s finally getting some cult recognition with the advent of the internet.