R.E.M.
Nick Drake
U2 (more so their later material, though)
Joni Mitchell
(some) Bob Dylan
Joanna Newsom
Tori Amos
The Velvet Underground (mainly the Nico tunes)
Death Cab for Cutie
Snow Patrol (the ones I like of theirs)
most of Radiohead's material
Oasis
almost all Pearl Jam songs
The Beatles/John Lennon
early Springsteen (even the "harder" stuff, like I guess maybe "Born to Run", has a way of calming me - I guess its just that epic to me!!
)
Regina Spektor
José Gonzalez
Van Morrison
Barenaked Ladies
Better than Ezra
old-school Counting Crows
Peter Gabriel
some Pretenders songs
Gin Blossoms
Toad the Wet Sprocket
old school Dave Matthews Band
Goo Goo Dolls once they became "mainstream" - but just the '90s stuff (though they've got some great "bad day" songs from before that happened, like "Long Way Down" and "Naked")
some Police (i.e. "Walking On the Moon", "Tea In the Sahara", "Walking In Your Footsteps", etc.)
The Shins
Belle and Sebastian
some Elvis Costello (i.e. "Alison", "Accidents Will Happen", "Everyday I Write the Book", and "Veronica" are all good examples of Costello's more "calming" side, to me)
Rickie Lee Jones
Cat Stevens
acoustic Neil Young songs
Guster
Pete Yorn
Feist
Jeff Buckley
Damien Rice
Elliott Smith
some Beck (like "Lost Cause" and "Orphans", for instance, or "Girl" and "Think I'm In Love" for a "happier" mood
)
10,000 Maniacs
the more acoustic side of Ryan Adams (though for his "electric" songs, "Burning Photographs" and "So Alive" also calm me down
)
Leonard Cohen (even his "eerie" songs, like "Everybody Knows", tend to have a calming quality for me)
Rilo Kiley
of course there's TONS more I could add to this list, but I think you'd get the general idea by now 
Last edited by GriffinGuitar12 on 25 May 2010, 3:09 pm, edited 1 time in total.