While many old pentiums will not run current versions of Windows, they
will run LInux without complaint. In our high school we have a cadre of
students who queue up to take our "retired" computers and move them to
Linux.
I recently saw a neat Beowulf cluster (a parallel Linux version) running
on a bunch of retired machines at Nebraska Lincoln. The cluster was put
together by a UNL undergrad as a "cool hack" and other students are
looking to run cosmic ray shower simulations on the cluster.