Dav Glass has probably become one of my favorite people on the Internet. The work he's done in the last year or so to make the Yahoo! User Interface library more of a community driven project has been really awesome, between the new forums, the transition to GitHub, and now reaching a new level with the YUI3 Gallery.
I've been using YUI3 since shortly after it was released, quickly recognizing the benefits of YUI3's advanced architecture over YUI2, going so far as to submit a bit of code, in the form of the collection module, and the work I've done on the Chroma-Hash module for YUI3. And that's just the code I've made public.
The best part of the gallery is that it's a dead simple way to make modules widely available for use, even if those modules will never really make sense in the context of the YUI framework itself, like my Chroma-Hash module, Luke Smith's Konami Event, or Dav Glass' quick and dirty port of the Simple Editor, but still might be useful to other people today. Plus, it's an incubator for code that might someday make sense in the core (Dav's YQL module being an example).
Yahoo!, and specifically the YUI team, have demonstrated exactly how a corporately backed open source project should run, as well as being a great example of why git (or really any distributed source control system) is so great for collaboration, especially for a growing team.
And it's just one way to contribute. Dav does a great job outlining all the methods that exist today, and some still to come