I'd like to add a quick word for Tcl. :-)
TCL is, even more than Perl, a very shellish language. If you don't come from a shell background, it can be confusing; for those used to more traditional programming languages, questions like "When do I need [...]? When do I need $'s on my variables and when don't I? When do I have to call expr?" have non-obvious answers. I don't think it's a good second language for an AppleScripter.
2. Excellent resources for GUI programming. Tcl's Tk GUI tookit
... is fully usable from Perl, Python, and Ruby. Sure, TCL had it first, but primacy isn't everything - or are you still using Mosaic to browse the web? :) It is true that the most comprehensive Tk documentation is still written from a TCL perspective, and the details of translating it to other host languages are sometimes a bit cryptic, but nevertheless the full power of Tk is at your disposal no matter which language you choose to work in.
5. A nice, supportive user community. comp.lang.tcl and the wiki at
http://wiki.tcl.tk are excellent resources.
