» Getting Horizons Horizontal :: Digital Photography School:

It's one of those simple little things that can really spoil an image - getting the horizon (which, unsurprisingly, should be horizontal) a couple of degrees off-line.

Fortunately, most D-SLRs have useful little lines in the viewfinder, and paying attention to these can make a big difference. However, we all get rushed, and make mistakes - so, as the article points out, it's really handy that tools like iPhoto and Photoshop have the ability to help is straighten these things out.