Here are some choices:

Morphix (which appears to have a new address) uses 2.4.21
http://morphix.netmaniacs.org/modules/news/

Cluster Knoppix uses 2.4.21-rc7 which I think is actually identical to the final 2.4.21 (someone correct me if I'm wrong). Cluster Knoppix is pretty much the same as the default, but with the additional clustering geatures.
http://bofh.be/clusterknoppix/changelog.php

Adam