The Deutsche Börse Systems uses TCL for performing functional tests of their application programming interfaces for all of their trading and clearing systems, i.e. EUREX, XETRA and CCP.
They have a TCL-based test application which is actually an extended TCL interpreter which can send all requests and receive data just as a real 3rd-party trading/clearing application does. They use a pretty smart approach which enables them to automatically re-generate their test application based on updated interface specifications, so they usually have zero to negligible efforts for maintenance of the test application itself.