Tandem Exchange (with Max) is a free site for matching up language tandem partners, both in person and over Skype or other remote tools.
German for English Speakers is a free online textbook and reference that explains German grammar through direct comparison with English.
Colibri is a web/mobile app for rapidly building vocabulary in a new language, currently in closed beta. See my presentation here for more information, or get in touch if you'd like to try it.
I have a blog with no particular focus and a few other occasional contributors, and a Tumblr where I collect passages from whatever I'm reading. And then there's Similar Songs, which is exactly what it sounds like.
I'm also a co-organizer of the Berlin Quantified Self group. Click here for more information on our meetups, or here for more on the global QS movement.
For the data from my meditation presentation at the 2013 QS conference (and on Seth Roberts' blog), click here (Excel link)