The compactly supported cohomology groups of the locally symmetric space for GL_n(Z) form, as n varies the E_1 page of a spectral sequence.  This spectral sequence was introduced by Quillen in 1973 in his proof of finite generation of K-groups, but also seems independently interesting as an organizing tool.  I will discuss joint work with Brown, Chan, and Payne on algebraic structures in this spectral sequence and a relationship to certain "graph complexes" introduced by Kontsevich in the 1990s.