Here is a convenient zip file containing all the T&S collabs I have, including the two most recent: Jonathan Coulton feat Sara Quin - Still Alive (Portal theme song cover) and Astronautalis feat Tegan Quin - Contrails. A lot of these I just ripped from Youtube myself.

These are all the songs I know about that I could find. If there are more out there, please let me know!

Full list:

Against Me! feat Tegan Quin - Borne On The FM Waves Of The Heart
Alkaline Trio feat Tegan Quin - Wake Up Exhausted
Astronautalis feat Tegan Quin - Contrails
David Usher feat Tegan Quin - Hey Kids
Dragonette feat Sara Quin - Okay Delore
Fences feat Sara Quin - My Girl the Horse (Accoustic)
Fences feat Sara Quin - My Girl the Horse
Jim Ward feat Tegan Quin - Broken Songs (Acoustic)
Jonathan Coulton feat Sara Quin - Still Alive (Portal theme song cover)
Kinnie Starr feat Tegan Quin - La La Le La La
Last days of April feat Tegan Quin - If (Don’t Ever Blame Yourself)
Melissa Ferrick feat Tegan Quin - Never Give Up
Rachel Cantu feat Tegan Quin - Saturday (2008)
The Reason feat Sara Quin - We’re So Beyond This
Theophilus London feat Sara Quin - Why Even Try
Tiesto feat Tegan & Sara - Feel It In My Bones (Original Remix)
Vivek Shraya feat Sara Quin - Your Name
Vivek Shraya feat Tegan Quin - The Alphabet

Forgot one! Updated to include Jim Ward feat Tegan Quin - Broken Songs (Acoustic). Thanks to waitwhatwho!