This is a great little museum that is a must see in Freiburg. It's located in the Augustinerplatz on Gerberau street. It has an amazing display of religious art and artifacts that were preserved by the monks over the centuries (it was previously a monestary).
Some of the pieces date from the 14th century, including a beautiful cross (meant to sit on an altar) made of gold and a gorgeous tapestry. There are also remarkably well preserved stained glass windows dating from 1300 and many more things.
It's free (unless you want to see special exhibits in which case it's 2 Euro for adults, kids 7 and under are free).