Github and QUnit
This year, Glow switched over to using Github as it's code repository when it went open source to make collaboration easier. To be honest, getting used to git and github in general has been somewhat of an uphill battle for the team, but we've really found it's come into it's own recently now that we've adopted using submodules.
Glow 2 now includes Sizzle, Woosh and QUnit all as submodule projects of Glow, and doing so has instantly made Glow feel like a more team-playing product.
The whole many hands make light work approach is really quite good fun. For example, Jake started working up a design for Woosh, his new speed test framework for JS that he's also open-sourced, which was making default QUnit look a bit... uh... under-loved, so I forked QUnit and started to reflect the designs, and over the last couple of weeks worked it up to something that the QUnit team have decided to accept back into the main project. Github made that extremely easy and now Glow, jQuery and the wider JS community benefits from a little bit of time spent making something that was essentially made to please just 3 pairs of eyes.
Picture credits: QUnit Screenshot from John Resig's Flickr Stream, designed by myself.