Straighten and Crop with the GIMP
One of the more awkward tasks to do in the GIMP is straightening horizons. You have to manually rotate the image using that tool which always seems to be fairly hit and miss.
I had hoped that GIMP 2.6 would have the “draw a line and rotate” function that Bibblepro and I think Photoshop have but unfortunately it’s not there. Not to worry because the Straighten and Crop plugin by Bert Hinz does the job too! (There’s another Straighten and Crop plugin too. I haven’t tried it. What’s it like?)
Install the plugin by copying the .py file into .gimp-2.6/plug-ins/. You might need to chmod it to make it executable. Fire up the gimp and using the Path tool (press B) mark two points on the horizon with left clicks of the mouse. Run the plugin (from Image->Transform->Straighten and Crop) and it will rotate the image.
Nice and simple and has worked on the couple of images I tried it on. (via)