Apple's free IDE, Xcode, only provides support for C(++), Objective-C(++), Java, Applescript and Makefile. Although it's possible to use a Makefile for other languages, I think it's more practical to fully integrate them through dedicated plugins.
At the time I wrote this page, Xcode already has a working plugin interface (it's even used by CoreData compiler/editor, CVS/Subversion/Perforce integration, GDB debugging…). However this interface is not yet public, because not really finished. According to Apple, it'll be public in future release but no real date is provided: developers just have to wait :-(.
You'll find below some plugins made after reverse engineering Xcode's plugin API, and a documentation of this API for writing third party compiler or linker plugins.
| Fortran Plugin: | download - doc - source code |
| Ada Plugin: | download - doc - source code |
| OCaml Plugin: | download - doc - source code |
| API documentation: | view |
Plugins made by other people (ask me if yours is missing):
| Haskell Plugin: | web site |
| CSharp Plugin (C#/mono): | web site |
| Symbian OS Plugin: | web site |
| D Plugin: | web site |
| Another D Plugin: | web site |