Web-based tools for grammarians: http://www.grammaticalframework.org/demos/gfse/
Ongoing work at http://cloud.grammaticalframework.org.
Look into online IDE platforms, like Kodingen and CodeRun.
There is work for Ajax-based code editors, eg Ymacs, which could be useful since there is a GF mode for emacs already (where?).
The emacs mode can now be found in http://www.grammaticalframework.org/src/tools/gf.el (note by Aarne)
There is also a Mozilla project, Bespin, to build a web-based editor extensible by javascript.
Also - check Orc, yet another online IDE for a new language, using CodeMirror as editor.