I've retooled the 'Parser Playground' gadget as an extension, which lets us more easily edit it & keep a master copy up to date. The gadget on mediawiki.org now loads the JS files from the extension, from a SVN checkout on toolserver -- handy that! :)
The updated gadget integrates a little better into the WikiEditor toolbar system, though it's still young and primitive. There's also now a primitive in-place editing mode when using the PegParser: you can click on any selectable node in either the preview or inspector panes and get a dialog box with the reconstructed source of just the piece you clicked on.
When done editing, click OK and it re-parses and drops it back into the updated document. Spiffy eh? After a while this'll get replaced with the fancier editing surface systems that Trevor & Inez are working on, but this gives something to poke in the meantime. ;)
Features, next todo steps, and screenshots on the extension's page: http://www.mediawiki.org/wiki/Extension:ParserPlayground
Primary next steps will be getting some round-tripping test helpers in there, making it a little easier to plug a third or customized parser in, and getting automated tests running from command-line (probably using node.js). And of course actually expanding templates will start making things interesting. ;)
The actual PEG grammar and the intermediate structure still need a lot more serious work to go beyond these demo stages, but at the moment I'm more actively looking at fleshing out the API between the parser/renderer and its host environment.
-- brion vibber (brion @ pobox.com / brion @ wikimedia.org)