Lilys, Stiltskin (both had songs featured on Levi's adverts), Spin Doctors.#
The Byrds , The Doors - both highly critically rated but not known enough by the mainstream.
Crookers, Kissy Sell Out, Marques Houston, Stone Sour, Royksopp, Doves, Magic Numbers, Missy Higgins, Aqualung, Angra, Minus The Bear.
So many, many more I could list too.
Animals As Leaders, Machinae Supremacy, Enslaved, Watchtower (hell anything with Ron Jarzombek), Heathen, Insomnium (swedish death metal which doesn't suck and isn't from sweden, yes they're not exactly obscure but more people should know about them), Mercyful Fate/King Diamond (again same applies), Mr Bungle, Amplifier, and Quo Vadis.
Stone Roses. Never really made it mainstream. There first album was one of the greatest ever
Interpol, The Strokes, Biffy Clyro should've been before 'Puzzle', Brand New, The Maccabees.
Kyuss should have been absolutely enormous. Better than QOTSA, for my money at least.
British Sea Power.
And everyone should be aware of the Pixies.
Noah and the whale, Laura Marling, Beirut, Stars, Bright eyes, Death cab, the drums, the gaslight anthem, jay jay pistolet, the king blues, tommy reilly, the tallest man on earth, manchester orchestra, great lake swimmers....blahblahblah