I've been reading Hatcher's exposition of the Madsen-Weiss theorem here. One of the key results needed is the group completion theorem. Hatcher gives several applications of the group completion theorem (the Barratt-Priddy theorem relating the stable cohomology of the symmetric group to the stable stems, the description of the loop space associated to the stable braid group, the Madsen-Weiss theorem, and Galatius's theorem on the stable cohomology of automorphism groups of free groups). Looking around the internet, I have not been able to locate other applications. The above theorems are all proven in kind of the same way (using "scanning"). Can people provide other interesting applications of the group completion theorem?

This should probably be community wiki, but I can't seem to find the box to check to make it so.