California Dreaming is easy? For heavy metal fans? Zowie.

One of the tips I've read about preserving your voice is: don't sing in a heavy metal band. :-)

Along the same lines though, here are a few that I'd say have already become folk, the criteria being that there are two or three "definitive" recordings out that what groups sing is usually something in between:

The Lion Sleeps Tonight
Me and Bobby McGee
The First Time Ever I Saw Your Face
I'm So Lonesome I Could Cry