One of the things that used to drive me nuts back in the film days was going out and shooting a bunch of nice landscape photos only to find that I hadn't been careful enough about keeping the horizon level. And nothing will make you queasy (photographically speaking, at least) faster than a horizon that slants down at one end.
The truth is that in the old days when all you had to compose with was a peephole viewfinder, getting a crooked horizon was common--most of those viewfinders (except in the most expensive SLR bodies) were not at all accurate. Things are quite a bit better with digital cameras since you can either compose using the LCD (which is far more accurate in terms of composition) or, if your DSLR doesn't have a live LCD viewing feature, you can check your composition immediately and make the necessary corrections.
Even with the LCD feature, it's not always easy to get a perfectly level horizon. I shot the image here from the deck of a ferry boat in New York harbor and the sailboats came into frame from around the sides of the ferry, so I had absolutely no warning. I fired off three or four quick shots and in a matter of seconds they were too far away to continue shooting. As you can see in the top photo not only is the horizon sloping downhill, but the buildings are way off kilter (if you had to walk down the halls in the buildings like that you'd think you were on a boat.)
Fortunately, fixing a crooked horizon (or crooked vertical lines) in most editing programs is a snap. Most programs have an easy-t0-use tool designed to do this. In my RAW editor in the full version of Photoshop, for example, all that I have to do is trace the offending horizon line (or a vertical line that is on tilt) using the tool and the program straightens things up automatically. There are similar automatic tools in most editing programs. But in Photoshop and Photoshop Elements (and I assume most other editing programs) it's just as easy to do it manually. Under the Edit menu you'll find a Transform mode and under that option you'll find a "rotate" option. To use the tool, just select the entire image area (Command A in a Mac; Alt A in PC-land) then navigate to the rotate tool (Edit>Transform>Rotate) and a little bent arrow appears. Click on that arrow and turn the image until it looks level.
There is a grid overlay tool in most editing programs that guides you in aligning horizontal and vertical lines but I find that it's just as easy to eyeball things if you have a strong line. Once you're satisfied with your correction either double click (Mac) in the image or click on the cropping tool once to accept the changes. Once you've got the image level you will have to crop the image to get rid of the dead space created by the rotation. Now simply save the file (giving it a new name, just to be safe) and you're done.