This script generates a poster like the famous Che Guevara one from Jim Fitzpatrick.
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Artistic -> Che Guevara.

See also the Obama "HOPE" Script.
Example here:



Update1: Added smoothness and contrast option.
Update2: Added lines smoothness and edge detection option.

For all scripts together, go here.