  The special linear groups, the mapping class groups of surfaces, the outer
autormorphism groups of free groups appear in numerous domains. Their
analogies, developped in particular in K. Vogtmann's work, have been written
about a lot. In this report, we concentrate on the contractible spaces on which
these groups act in an analogous way, on the common properties of their
subgroups, and on the similar (or conjecturally similar) properties of their
asymptotic geometry.
