I would add MathMap
http://www.complang.tuwien.ac.at/schani/mathmap/

Is free, available as standalone for MAC
Or in Linux and Windows as a Gimp plugin

As the mame indicate is ideal for mapping several mapping filters are included other available here
http://photocomix-resources.devianta...cript-74832004

and for edit equirectangular images here

http://photocomix-resources.devianta...t-2-1-83507321

other filters may be easily created (well "easy" is subjective, let say easier then write a correspondent C code
or Gimp script).

Linux version is much quicker, while the Windows last build
http://photocomix-resources.devianta...ndows-95950939
included the extra scripts quoted above.