"Tcl is part of Cisco's IOS router operating system, and as such is included in all high-end Cisco routers and switches."
What's that? Tcl on my router? A quick Google for "tcl cisco ios" revealed two helpful resources: Cisco IOS Scripting with Tcl by Cisco and TCL'ing Your Cisco Router by Peter J. Welcher. Cisco's document revealed that Tcl 8.3.4 was introduced in Cisco IOS 12.3(2)T. Sure enough, it's on my Cisco router:
Cisco IOS Software, C2600 Software (C2600-ADVSECURITYK9-M),
Version 12.3(11)T3, RELEASE SOFTWARE (fc4)
gill(tcl)#puts "Hello world"
Peter Welcher's article expands on Cisco's document to show some of what can be done with Tcl on the Cisco router.
This absolutely blows my mind, and I can't believe I haven't heard of this before. I found a 2001 email mentioning the Tcl shell as "undocumented," although there is a reference in the Cisco IOS Configuration Guide Master Index, Release 12.1. Imagine what might be possible with this; you might be able to run arbitrary code -- Sguil even? -- on Cisco routers.
Speaking of Tcl, the newest versions of Salvatore Sanfilippo's Hping rely on the language. I reported on this last year. Salvatore has been working on a Tcl interpreter called The Jim Project, which he says "will likely merge with Hping." Salvatore is also writing a book on Tcl, some of which is online as TCLWISE: An introduction to the Tcl programming language.