cross-platform rendering

i second what colourless said, and not only for libraries, but it could hold for drivers also, depending on the driver model of the particular os. but for apps it's ok, i'd say.

now, if you're absolutely positive that you want your library/driver to stay open-sourced forever and not be used by a closed-source project, then go for GPL. otherwise pick any of the LGPL, MIT or BSD and likes licences.