Windows: Perspective Image Correction is a free Windows utility that will correct photos you've taken where the subject -- like a building or monument -- is out of alignment, or appears tilted. The app even gives you the ability to select the area of your photo it works on, so it doesn't inadvertently adjust a person standing in the foreground.
Obviously, the best way to make sure the perspective in your photos is correct is to take time and line up your shot before taking it, but if you're taking tons of photos or shooting from the hip, it's inevitable you grab have an shot or two that's just a little off. Perspective Image Correction can fix it. When you load an image, the app prompts you to select the correction area by setting four alignment points around the object you want to straighten up. Once you do, the app automatically fixes the area inside those points and shows you the result before saving the finished product. If you don't like what you see, you can do it again, or zoom in and correct smaller areas individually.
Perspective Image Correction is free and available for Windows only, and you'll need .NET 3.5 installed for the program to work. You can grab it at the link below.