I love the Jar of Hearts song by Christina Perri, too. I found out about it here on SR, I think.
Call Me When You're Sober by Evanescence also
Hit 'Em Up Style by the Carolina Chocolate Drops - another one I heard here at SR
I'm All Over It Now - Jamie Cullum
Summertime - Ella Fitzgerald - just anything by Ella
Fresh Feeling - by Eels (I think)
Give Me One Reason - Traci Chapman
The Last Day of Our Acquaintance - Sinead O'Connor
And not really uplifting, but it just cracks me up: Tangerine Speedo - by Caviar