The Perspective plugin lets you carry out both active ("Modify") and passive ("Rectify") transformations. You can rotate the image around three axes, then reposition and resize it. Grid lines can be displayed both before and after transformation.

History of Changes:

Version 8.2 (current version)

    Help button and tooptips option

Version 8.0

    Support for 16 bit/channel images
    Support for grayscale images

Version 7.9

    Support for left-handed mouse settings

Version 7.8

    Snap function for easier positioning of control lines
    Selectable control lines color
    Resizeable dialog
    Minor bug fixes

Version 7.5

    Use shift key to magnify region of interest for accurate placement of control points
    New user-friendly Crop section replaces previous Positioning section

Version 7

    Set control points in the preview image. The plugin evaluates these control points and sets the slider values accordingly to rectify the image.
    Choose between various rectification modes
    De-barrel slider to compensate barrel or pincushion lens distortions. Comes in handy for architecture photography.
    Larger GUI and image preview
    Finer slider increments for higher precision
    Track option to enable/disable instant preview
    Automatic positioning works even with non-zero rotation
    Various minor bugs fixed

Awards
Perspective Transformations got the editor's recommendation in a test by DigitalPHOTO.