Update 10.6.2011 - the new version of the Straighten and Crop script (attached) will use the Python version of Post Rotate Crop if this is installed; otherwise it will use the Script-Fu version of the script. (The Python version of Post Rotate Crop is very much faster than the Script-Fu version)
A script that provides an easy method for straightening pictures and then, optionally cropping them to remove the resulting transparent areas.
The centre of rotation can be specified as can the type of cropping required.
The cropping can be performed by the GIMP rotate function but this can have unpredictable results. The Post Rotate Crop script provides more options and seems more reliable (let me know should you find a problem with it)
This version of Straighten and Crop uses the revised interface to the Post Rotate Crop script, including the option to select the aspect ratio of the cropped area.
The attached zip file includes the script and a text file of notes on its use.