Some scans for some reason refuse to go in level no matter how many times you play with the original artwork in the scanner or no matter how careful you are holding the camera without a tripod.
Here, using Photoshop is a quick and simple way of getting them level. I'm using Photoshop CS2 and can't remember which version this feature was introduced.
Right-click on the eye dropper tool and from the pop-up menu select "Measure Tool".
http://img155.imageshack.us/img155/6765/stage1gx3.gif
Now, on a clear part of the picture (ie. following the horizon) draw a line from one point to another point. Now it's best to make this line as long as possible to make the rotation as accurate as possible.
http://img157.imageshack.us/img157/227/stage2bf8.jpg
(Note the line following the top of the text)
Next from the "Image" menu select "Rotate Canvas>Arbitrary" and a small window will open and if all has gone as planned there should be a number already entered which Photoshop obtained from your guide line.
http://img143.imageshack.us/img143/2567/stage3vl7.gif
Click "OK" and the image will then rotate. All that's needed now is to crop the image to remove the extra space that was added around the picture during rotation.