GIMP is the GNU Image Manipulation Program, a sophisticated cross platform image editor. GIMP is Free Software.
The reverse transform setting enables the following GIMP workflow for retouching a panoramas tricky sky/ground as well as any other arbitrary views:
- Duplicate layer (and add alpha channel)
- Set pan/tilt/zoom of virtual camera and apply transform
- Do retouching/repair
- Re-open the panorama-projection filter, recall last settings and enable inverse transform.
The layer now contains a patch that can be further refined/blended before being merged back down to the panorama.
- The build in "Little Planet" an be used to create "Little Planets" from 360x180° panoramas. 
- pandora plugin allows you to spread a series of photographs into a multi-layer image for easy editing.
- phfluuh plugin arrange photographs into a multilayer image, correct barrel distortion and map from rectilinear Projection to cylindrical Projection.
- Stitchpanorama plugin lets you pick control points and distort photographs to fit.
- Save Layers in separate files. Little Helper for Multilayer TIFF. If you are using multilayer tiff with GIMP for optimizing blending with enblend, you might need this script for saving every layer to a seperate file. Download continous_save.zip
- save-layers-tiff is a GIMP script which makes it easy to edit multi-layer TIFFs and send the results to enblend; see Multi-Layer TIFF editing with GIMP.
- resynthesizer is a very useful plugin that can be used for filling-in missing parts of stitched images.
- Refocus-it, a plugin for sharpening images based on a minimum of the error function using a Hopfield neural network.
Notes for panoramic imaging
A single layer of an equirectangular panorama generated using standard techniques can easily consume more than 100 MegaBytes of memory, whereas the default GIMP cache size is more suitable for single photographs. So to speed things up for panoramas, in File -> Preferences -> Environment -> Resource consumption change Tile cache size to a significant portion of your available memory.