On the off chance you're not already familiar with YSlow, it is used to analyze Web pages against a set of best practices. YSlow crawls the DOM of a page, gets information about all of the page's components and then generates a grade for each of the 23 performance rules. YSlow has extensions for Firefox and Chrome, and has bookmarklets for Opera, Safari and mobile devices.
YSlow project lead Marcel Duran says that developers are "encouraged to use the source code, learn how it works, fork it to make your own projects and enhance it with new rules, features and whatever will improve this tool we all love."
YSlow's GitHub repo was announced late last week, but already has 44 forks and 538 watchers. It'll be interesting to see how it evolves now that Yahoo is throwing open the doors to community contributions.