If you're like me, you're a little off-center already and sometimes your photos come out a bit crooked. A friend informed me that Picasa has a straightening tool, so I downloaded Picasa solely for that purpose. Photoshop will do it as well (of course; I mean, what are we paying for, right?).
Several of my Pelican Island photos were tilted, like this one:
But with a little magic, all is now well:
Again, though, some information loss with the edit; the original file is 4.5MB, the straightened one is 4.0MB. The best way to fix that is to take a straight picture the first time. I have since been looking religiously at the LCD screen to make sure, and that way I can snap a new, correct photo while I'm on the scene.