Kate Bush: She is /awesome/. I've been a fangirl of hers since I was twelve. x33 The Kick Inside is one of my favorite albums, and some of the songs touche a number of pretty taboo subjects. Not everyone likes her voice, but I think she's rockin' awesome, and an surprisingly large amount of people in the music industry today (mostly indie songstresses) actually cite her as a big inspiration.
America: Apparently they were, in fact, pretty big in their heyday, but these-days everyone just thinks of "Horse With No Name", but they've got lots of other songs, better than their most well-known hit, and their lyrics are pretty well done at times. Not everyone's into folky soft-rock, mind you. But I luff them. 8D
Emerson, Lake & Palmer: I wouldn't recommend these guys to everyone, because even though they do have a fairly big following they're certainly not everyone's cup of tea. Tortured moogs, anyone? But seriously, all three guys are extremely talented. Even though egos did seem to get a little in the way at times (hey, they're in the music industry- what do you expect?), I kinda wish more bands these-days were as ambitious as these guys and just did what they wanted, regardless of what's fashionable.
Cream: I'm almost certain that everyone knows "Sunshine of your Love", but they've done a lot of other great songs.
King Crimson: Pretty much the same reason as to why I find ELP underrated. These guys do have many fans out there, but people don't really acknowledge how influential this band was.
Atomic Rooster: I think they came out around the same time as Led Zeppelin- and they're just as heavy. I really wish these guys were more well-known, they produced some really rockin' stuff.