Author of this application is Tom Poindexter.
sockspy [L1 ] "watch[es] the conversation of a Tcp client and server", according to its quite readable README [L2 ]. Although "not a replacement for heavy duty tools such as 'tcpdump' and other passive packet sniffers" (Ethereal [L3 ], ...), it's easy to install, configure, and launch, and pleasingly convenient for "debugging Tcp client/server programs, examining protocols and diagnosing network problems".
sockspy is nearly indispensable for any network programmer or network administrator, quite apart from any interest they might have in Tcl. [Compare to tcpdump, ...]
sockspy's a Tk application. CL has been thinking of refactoring it to expose a pure Tcl part. One benefit might be that at that point, the code could be considered as a quick-assemble toolkit for proxy servers. Write [L4 ] if that interests you.