This is a long list, so be prepared (all different types):  Jeff Buckley, Blackmore's Night, Nightwish, The Matches, Stargasm, Angel(glam rock band, not easily searchable, but amazing), Avantasia, Edguy, Stratovarius, The Libertines, Mogwai, My Bloody Valentine, Queensryche, Nick Drake, Patti Smith, Phish, Portishead, Screaming Trees, Mudhoney, Smashing Pumpkins(which they should already have and shouldn't need to be recommended), Sonata Arctica, Sweet, Tortoise, The Velvet Underground, Yngwie Malmsteen, Dave Matthews Band(another one that shouldn't need mentioning), Ben Folds

Also, the soundtracks for: Empire Records, Juno, Pulp Fiction, Almost Famous, and The Royal Tenenbaums.  Oh, and Trainspotting. 

Music is life.