Posting this handy Gimp plugin here, just in case it drops off the Internet. It was licensed under CC0 by the original author.
Download ez-perspective.py here.
Installation: copy the Python script into your plugins folder (location can be found under Preferences > Folders > Plugins) and restart Gimp. There should now be a new menu item under Filters > Distort.
Description, taken from the file:
EZ Perspective
A Gimp plug-in to easily change the perspective in an image, just like it says on the tin.
Particularly useful for correcting photographs, but can also be used to add a Dutch angle to any image.
Provides 3 parameters to change up/down (tilt, pitch), left/right (swing, yaw), and rotation (twist, roll).
Please use:
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after correcting lens distortion (barrel distortion, vignetting, lateral chromatic aberration), as these depend on the lens, on the shot as taken
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before cropping, as this changes the shape of the image, and will likely warrant cropping afterwards