Well OK, here's a few at random that I like that aren't all that mainstream I guess, although they are pretty mainstream on the Moon:
Rain - Cerulean Blue
Frost - Milliontown
Pilotdrift - Water Sphere
Ozric Tentacles - Erpland (or any album really)
Happy The Man - Beginnings
National Health - Complete
Jade Warrior - Waves or Way Of The Sun
One Shot - Ewaz Vader
Pineapple Thief - 10 Stories Down
Group 87 - Group 87
One man's mainstream is another man's obscure of course.
These are all over the map style-wise but are all on the Moon, I'd request a tune or two to check the albums out before buying.