I've already recommended these for somebody recently...Mendelssohn wrote a whole book of duets (many originally for Victoria and Albert!) which are really good.
Ernest Chausson wrote a couple of fab duets which I've done with a tenor (though it could also be 2 sops) - they're seriously challenging!
If you want slightly lighter stuff, don't forget the Gilbert & Sullivan repertoire, as well as some of the other operettas (Strauss, Offenbach).
Jay Althouse arranged a whole book of folk songs for 2 voices. Not sure if it's still in print but they are really nice arrangements.
I must make a list of duets - I usually seem to be the one who suggests things when this question comes up!