Only some of the latest pet projects are featured here, see the section below for the full list.
Lots of good projects that use Mercurial migrated from Bitbucket to foss.heptapod.net.
If you wanted to see some fancy GitHub-like chart of my activity, this is currently the best place to do so. Not that such chart would be indicative of anything, but sometimes people ask anyway.
One of the most famous open source projects that I contributed to is this. Not only my favorite VCS, but also a great project to learn new technical things and workflows. I'm also a reviewer for hgweb.
Projects hosted here also get my attention, but not that often.
Open Hub is a free public directory of open source software and people. It can do pretty graphs with highlights and code analysis (including evaluations like “Very few source code comments”).
It's similar to Project Euler, but most of the problems are more bioinformatics-related than maths-related.
Since my preferred VCS is Mercurial, I used to put my code here.
Also, a big chunk of ExtJS experience comes from this project (this link might be dead now).
Improving localization is essentially improving user interface. When I was just starting to contribute to open source, I used Rosetta, which is Launchpad's translation service. And it still tracks my translations, diligently, even though I don't use the system. So it gets credit here.