This tool is useful for would be architectural photographers, but may help anyone out who has an image that looks a bit unatural due to a low viewpoint (it happens a lot). Gimpshop has a specific tool to correct it quite easily.
For this task, we'll use the picture of the Council house in Nottingham.
With the Council House image open, one really useful tip is to zoom out of the image to leave some area around it. This will make it easier to see if your correction is OK.
Step 2 - Select the Perpective tool 
With the image open, select the Perspective Correction tool, highlighted by the red box on the left.
Ensure that the whole image is selected by using the menu option: SELECT > SELECT ALL
Then click and hold on a corner point of the image. The Persepctive dialogue will appear (you can ignore it for now) and you can hold and drag the corner point of the image to 'distort' it.
Look at the verticals of the columns. Make them align to the vertical 'edge' of the image.
To complete the image, adjust the left hand coner as well. When you are satisfied with the result, you can crop the image. This is done by ticking the 'Clip result' box in the Tool Options part of the Tools palette and then using the 'Transform' button in the Perspective dialogue box (below).
Again, use FILE > SAVE AS to save your edited image as a new file.