He Can play much positions,he can be DM,CM,at the WC 2002 he was RB,and in Borussia Dortmund he was a kind of OM. But I think nowadays in Bremen he is more a destroying DM.
In Bremen he naturally plays RM. He's been playing that position since before he went to Bayern and does it when the rest of the midfield is fit.
Normal Werder midfield (no injuries):
----------------------Diego----------------------
------Borowski----------------Frings----------
------------------Baumann (C)----------------
Now that both Borowski and Baumann are missing, Frings often plays DM even in Bremen like he was doing with the NT. It looked mostly like this this year:
---------------------Diego-------------------------
---------Jensen--------------Frings/Vranjes-------
-------------------Vranjes/Frings----------------
or like this (with Borowski):
---------------------Diego-------------------
--------Borowski------------Jensen---------
--------------------Frings--------------------
His best positions are definetly DM and RM. I personally like him better on the RM position because then he shows that he can actually give a lot of creativity to the team, plus he can do some decent crosses, one of his abilities not too many people know about.