test2.wikipedia.org
test2.wikipedia.org (created in 2011) is a test wiki inside the production cluster (like test.wikipedia.org). It was originally created to ensure testability of all Het deploy states throughout the deployment phases (where test.wikipedia.org would be the first wiki to get the next version and test2.wikipedia.org the last – together with the Wikipedias).
As of mid-2012 test.wikipedia.org and test2.wikipedia.org both get the the "next" wmf-branch in the first phase.
Details
Though test and test2 are both served from the production cluster, test2 also runs on the load balancer serving live scapped code from the apache clusters. test, on the other hand, is served directly from an isolated Apache server.