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:
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).
after correcting lens distortion (barrel distortion, vignetting, lateral chromatic aberration), as these depend on the lens, on the shot as taken
before cropping, as this changes the shape of the image, and will likely warrant cropping afterwards