A while back, I discovered a very useful tool in Gimp to straighten images. This is useful if you have a bunch of scanned text (its advertised usage) or if you have what I have: a group of pictures that were scanned in on a flatbed scanner and all 2-3 degrees off of horizontal.
Since upgrading Ubuntu, I’d lost track of that plugin. I started by searching for “straighten picture” and that led me to the Straighten and Crop plugin: http://registry.gimp.org/node/18821
That’s a useful tool, but not what I wanted. (It lets you pick two points on an image—say, the leftmost and rightmost horizon lines—and rotate the image so those two points are even.)
Eventually I figured out the tool I wanted was Deskew (aka auto-straighten). It can be found here: http://registry.gimp.org/node/2958
I downloaded the .deb for 1.1 and installed it in my Ubuntu 9.10 with:
sudo dpkg -i gimp-deskew-plugin_1.1_i386.deb
Once it’s installed, it’s not immediately obvious where in Gimp it goes. (And of course the web page is no help.) The secret location is:
Filters -> Misc -> Deskew
Load an image, select that, and the image will be straightened. Then you can crop it and save it; nobody will ever know it wasn’t straight on your scanner.