Edrx's Icon stuff
(2005sep09: below this point things are really really old. Expect broken links.)
Icon is the best language I've ever met. It's not as fun to program in as Forth and it's not very good at interacting with the system, but.
I once wrote a program to convert deduction trees in ascii-art to TeX code, and I still use it a lot; I don't have time to document it now, though. :-(
There are two languages that I've been using much more than Icon lately: Tcl (and Tk, and Expect) and Lua. Tcl is beautiful but slow; Lua has an ugly syntax but some nice concepts behind, and is very fast. I'll write a decent comparison between the three someday.
The main book on Icon, "The Icon Programming Language", by Raplh and Madge Griswold (unbeatable! The best programming language book ever! Etc! Etc!) is now online; go to http://www.cs.arizona.edu/icon/ftp/doc/ and fetch either lb1up.pdf or lb2up.pdf.