I forgot to mention that it is the Picture Viewer. Let me explain one more time I may not have been clear enough.
You have a page from a book that you scanned and it is maybe 10-15 degrees off from being straight. In other words the picture is cocked. Then on that page you have a title and that title is "This Is the Story of Computers." What you would do is draw a line under "This Is the Story of Computers", clicking A would cause that page to rotate to a perfect picture view. If you click B that page would rotate to a perfect landscape view; in this case that would be an undesirable choice. But is some cases that would be a desirable value.
The Opus viewer only does 90-degree rotations at the moment so you can't do this in Opus directly.
You can definitely do it in Photoshop as I do it all the time with photos to line them up. (You use the Ruler tool to draw a line, then use Image->Rotate Canvass->Arbitrary and the dialog that appears will already have the correct amount of rotation specified which will make that line horizontal or vertical (whatever is nearest), so you just push return and it's done.)
Of course, if this is all you want to do then Photoshop is expensive overkill when Paint Shop Pro can do it.
While we are talking about graphics software, I'd like to mention Mediachance Photo-Brush.
While not free (however low-cost) and lacks currently layers (to be added in the nearest free update it seems), it has some interensing and well-working image processing and painting features. I use it daily for my photo-work.