Alternative to GLUTesselator?

For whatever's worth, the SGI Free License B 2.0 is now GPL-compatible, being essentially identical to the X11 license, according to the GNU project:

http://www.gnu.org/licenses/license-list.html#SGIFreeB

http://www.gnu.org/licenses/license-list.html#X11License

The SGI Free License B 2.0 is the license under which the GLU tessellator in GLU 9.0.0 has been released:

ftp://ftp.freedesktop.org/pub/mesa/glu/

So go ahead and use the GLU tessellator in your GPL project!

There's always GPC.

EDIT: Some others:

Flipcode mystery triangulator. Slower than GPC in my extremely limited, probably wrong tests.

poly2tri is BSD-licensed.

EDIT2: Earcut.hpp is now a thing.