I know how big a deal the level horizon, or lines are in the images we post, so while looking through one of my Scott Kelby books on CS2, he lists a super easy way to adjust the horizon.
Once I did it his way, I was kicking myself for not knowing this trick earlier. It takes all the guess work out of how to get it level and CS2 does ll the work for you with just a click or two of your mouse.
For those that use CS2, I havent checked it in CS3 yet, or anything later as I dont have any of the older versions anymore, but it might be the same.
Open the crooked image in Photochop.
Right Click over the Eyedropper tool in the list of tools on the left of your screen. The third option is a tool labeled Measure Tool.
Click on the little ruler and go back to the image.
Once on the image, click on a section of the horizon and drag the line accross the horizon or edge of anything that is not level.
Then go up to the top tool bar, and click on Image.
Drop down to Rotate Canvas. A window will open giving you options to choose from. Click on Arbitary and Photochop will tell you how many degrees the image needs to become level and will know whether or not to rotate it clockwise or counter clockwise.
You might need to crop the canvas to get rid of the white edges around the image.
It's that easy.
Hope that helps. I know it's a trick I use with almost every image and it's sped up the process considerably