The Google Code Search team has just announced support for AppleScript, COBOL, ColdFusion, Haskell, Modula-2, Modula-3, OCaml, R, Rebol, SML, and VHDL. You can select them on the advanced search page or with the "lang:" operator. Other languages are supported with a bit more work:
If your language of choice isn't on there, you can still search for code written in that language. Just use the the "file:" operator to restrict your search to files with the right extensions. For example, you could restrict your search to files with a .zz extension with a query like:
Or, you could restrict to multiple extensions, for example to .x and .abc with something like:
hello,\ world file:\.(x|abc)$
For more info see the FAQ.