Force of Gravity - BT (ft. JC Chasez)
I Will - David Foster (ft. Chris Kirkpatrick)

Even though those aren't really duets. The artists provide the music and JC and Chris sing the lyrics.

Makin My Way from Fakin the Video - JC and various artists

The Rainbow Connection - Justin Timberlake and Kermit the Frog

If I Don't - Santana (ft. JC Chasez) even though we'll prolly never hear the whole thing, at least the clip is out there

I'd Do Anything For You - Trey D (ft. JC Chasez)
Just A Little More Time - Trey D (ft. JC Chasez and Kevin Richardson)
***not released on disc but dl'd from Trey D's MySpace***

Messed Around - Boyz n Girlz United (ft. JC Chasez)

That's all I got.