Like many Open Source applications, the Mozilla Firefox web browser has a huge range of extensions (or Add-ons as Mozilla likes to call them) which extend the functionality of the browser in all manner of ways for all manner of different audiences. What has happened is that the add-ons have become an indispensable part of the Browser Experience.
Now Mozilla have a new feature on the site where people can create and subscribe to Add-on collections. From a selfish point of view, this is good news, as I can use this service to manage my default set of workaday Add-ons, which I have shared indirectly and with others through a post on my ZDNet blog.
* Despite what it says on the Mozilla site, there is a linux version of HTML Validator available directly from Marc Gueury's site.