I frequently write about cross-language considerations in our "Regular Expressions" column on scripting languages. That is, we both comment on such ideas as event-based programming, Unicode, "push", functionalism, and others that apply across languages, and also cover specific technologies that involve co-operation between different languages. Titles in the latter vein include "It's a good time to be a polyglot", bindings of Tk and other GUI toolkits for several languages, [dozen more]. [Fill this paragraph with hyperlinks.]
[Important: if there's something you want ("How can I combine Prolog with Ruby?", for example) and don't see, please ask. I might well have a reference which I just haven't written up yet.]
Alex Martelli compares Jean-Claude's Minotaur and the .NET IL.
Jean-Luc Fontaine has created TclPerl and TclPython.
[Several interesting comparisons and conclusions arose from Perl6 discussions.]
[explain Zope-inspired Perl-Python system.]
Rich Kucera seems to be involved in controlling Win* (ActiveState) Perl with Franz Common Lisp. I'm not sure his work has come to any particular conclusion.
[Explain my XML-RPC sentiments.]
"The Inline module (now at release 0.26) allows you to embed code from other languages in the middle of a Perl program." Languages currently supported include C, Python (Inline::Python), and Tcl (Inline::Tcl) Also, Tcl is a Perl module, last updated in 1997, that allows for direct invocation of Tcl commands. There have been several efforts in the opposite direction; most reusable of these appears to be Artur Trzewik's dbitotcl. Tcl (Nautcl, and others) and Python (Weave) have analogues with both advantages and disadvantages, compared to Inline::.
[Explain connection to "scripting-within-scripting".]
[Explain Python and Perl modules for Tcl access.]
Live demonstrations of different languages afford the opportunity for "online programming". These are services I know set up to execute live code: