It's no secret that straightening the horizon on your image can really improve its composition and appeal. But how do you get it exactly straight? I do it with Photoshop or Lightroom. (CS3 and v.1.4, I have yet to update). So here's how I do it with Photoshop:
I took this on a hike at Morraine Lake in Banff Calgary but I didn't use a tripod so its slightly crooked. Not a big deal, just a little distracting. Nothing a little Photoshop can't fix ;) So open up that baby and let's get started! BTW, that blue line is just there to show how crooked the photo is.
First things first, duplicate your background layer (cmd+j or ctrl+j) so in case something goes wrong you still have your original image.
Next, go to Filter > Distort > Lens Correction...
In the Lens Correction dialog box double click the Zoom Tool to zoom into your image 100%. Zoom into your crooked horizon.
Select your Straighten Tool.
Take the straighten tool and draw over (trace) any part of the horizon that should be straight.
And boom! you're done.
The process is more or less the same in Lightroom. You can find the straightening tool in the crop menu. :)