pstree

Download and unpack:

wget https://mojzis.com/software/pstree/pstree-20140101.tar.bz2
bunzip2 < pstree-20140101.tar.bz2 | tar -xf -
cd pstree-20140101

Compile and install:

make
sudo make install

Or create Debian package and install:

dpkg-buildpackage
sudo dpkg -i ../pstree_20140101_*.deb