If you really need it to create good textures, no way :
buy it !

Juts my personnal advice !

you can do that inside of blender, you can create brushes from texture and tile the image, as well as, warp it to paint across seams. all from the image editor. It is easy to setup and easy to do...very straight forward. The only drawback is that I do not know how to do layers....someone will say it is not possible, but anything is possible if you plan for it and think about it.

Does anyone know of such a plug-in for GIMP? I've wanted one for a long time, although I'm willing to look into texture maker later on.

Jibb