"Don't Go Breakin' My Heart" by Elton John and KiKi Dee.
Wont back down.
My former wife and I used to sing "From This Moment On" in places.
Lucky by Jason Mraz is soooooo nice!
Islands in The Stream
Check out some songs that Glee has covered. They have quite a few good duets.
Time of Your Life, Written in the Stars (from Aida), Come What May, As Long As You're Mine (from Wicked), The Song That Goes Like This (from Spamalot), Just Give Me a Reason....that's all I can think of right now