There are several ways to straighten out a line in your image.
The two most common situations where you may need this technique
are squaring up an image that is slightly rotated (perhaps not aligned
perfectly at scan time) or to straighten out an errant horizon line.
Our favorite method of rotating an image is to use the ruler tool
in combination with the Rotate Canvas command.
Select the Ruler tool (which is now hidden in Photoshop 6 under
the Eyedropper tool). Holding down the mouse, select a starting
point on one end of as long a portion of the axis you want to rotate
along as possible and draw to the other end of that axis. The longer
the segment you choose, the more accurate your rotation is likely
to be. You also want to select a starting and ending point in which
you can clearly see the difference between the pixels on either
side of where you click the mouse. This is because you want to start
and end the ruler line at the same relative position along the line,
ideally almost to the level of a pixel.
Once you've drawn your line, go to Image->Rotate Canvas->Arbitrary.
When this dialogue box opens, it will automatically contain the degree
and direction of rotation needed to make the line you drew perfectly
straight vertically or horizontally.