First off, You do not need to have prior knowledge of blender to use this utility. (Yay! :) ) If you can save and load files from an application, you can use this :) .
This is a nice little template for blender that will let you see your textures tiled in a 3x3 grid, and let you paint anywhere on the grid in real time.
First off you will need Blender:
Install blender first, then download this file:
If you have blender installed , your browser will automatically offer to open it for you, or you can just save it to disk (recommended).
After you open up the file, you should see this:
Use replace image, when loading your custom images... Load image will not load it into the proper material block.
This should be really useful for fixing bad seams in seamless textures,
and for creating new ones from scratch!
Old time blender users, please make sure you have the most recent version installed to ensure a bug free experience :) .