As Derick Rethans notes in his latest post, the Xdebug project is now on github and can be found under his account.
I've just opened my Xdebug repository on github. Importing my old SVN repository was a bit of a task. With the help of Ole Marius Smestad I managed to cook up a bash script to import and fix-up the repository. In the meanwhile I've learned more about git than I ever wanted to know, but still not enough.
Besides the source for Xdebug itself, you'll also notice repositories for the Xdebug website, the GtkDBGp client and the DBGp protocol definition.