[UPDATE: At the original post where I read about Wingman, someone (named Douglas) said that the Html Validator for Firefox extension by Marc Gueury does the same thing as Wingman, and also runs locally, instead of sending the web page for validation to an external site, as Wingman does. So the HTML Validator extension may not have the same potential security issues as Wingman, that I and some other commenters on that post talked about. And this one is downloadable. So it can be tried out right now - by anyone, without waiting for an invite. I'm going to try it out. ]
Saw about Wingman here just now.
The Wingman site.
The developers of Wingman are Rowan Simpson and Koz. Rowan was a "top tech dude" (according to the O'Reilly post - nice way of putting it :) at TradeMe, "New Zealand's eBay", and Koz is a core Rails developer.
Wingman seems potentially useful for web application development, based on the description. And since it's a Firefox plugin, it should work regardless of the technologies you use for your web app - whether PHP, Python and Django, Ruby on Rails, JEE (J2EE), or any other ...
Note: there is no download link available as of now, the site is invite-only; looks like they notify you when you can access it (once you sign up for an invite). Not sure why they do that, unless it is to release it to a smaller set of users first, to get feedback from them, instead of getting too much feedback. Which might not be a bad idea ... And there being no download link is also why I put a question mark at the end of the title of this post - since I can't try it out (yet).
Vasudev Ram - Dancing Bison Enterprises.