Libertinage was commissioned for the FLOSS+Art book. It’s licenced under the OFL, which is emerging as the standard licence for fonts.
OSP say –
Libertinage is a remix of Linux Libertine and was designed in August 2008.
For more detailed information: ospublish.constantvzw.org
We built Libertinage by copying and pasting parts of Linux Libertine
glyphs. There are 26 variations, one for each latin letter in the
Libertinage.ttf is the ‘Full’ version, containing all modifications.
Single letter versions are named Libertinage-a, Libertinage-b,
Libertinage-c… depending on the letter that was changed. All 26 are
gathered in the Libertinage package as .ttf files.
‘La vie est triste comme un verre de grenadine’
Mylène Farmer, Libertine (1985)
OSP have an excellent blog on using Free Software for design work here.