I don't think we have made a list of what plugins we use so i will start.
I have a few plugins but i rearly use any other than resynthesizer. It is a texture synthesis, but it works good for making tilable textures.
I know there is one that folows GIMP, but i find this better.
You select what you want to make tilable, select horisontal and or vertical then start it.
It works inside the same window and i find the result more than ofter to be good.
project link: http://logarithmic.net/pfh/resynthesizer
GIMP registery page: http://registry.gimp.org/node/25219