Software: Photoshop CS3 (Should work in v5.0+)

NOTE:  As always save a copy of your image before making any adjustment.  It's easier to delete a copy than retake a picture.

Step 1
Select an image that needs to be straightened.  The image doesn't make any difference as this method can be used to rotate any image.

Step 2
Switch from the eyedropper tool to the ruler

Step 3
Use the ruler to create a line along the edge you want straightened.

Step 4
Select the image menu > Rotate Canvas > Arbitrary

Step 5
The rotation box will open with the settings needed to straighten the image.  Click OK

Step 6
Now that your image is rotated crop it as needed and proceed with any other corrections.