These are bzr branches. You can download them using the bzr branch command. If you open a branch in your browser, you will be forwarded to Loggerhead, a bzr web interface.

bitlbee/Main release/integration branch
wilmer/Personal branches
jelmer/Personal branches
maurits/Personal branches
contrib/Personal branches from others
Other stuff
debian/Debian packages (nightly builds of various branches)
tarballs/Tarballs (with prebuilt docs) from bzr
lh/Loggerhead, bzr viewer web interface