PTLens is an excellent application to correct or add distortion to your pictures.
You'll be able to apply a fish eye effect, modify perspective, chromatic aberation and vignetting.
PTLens also lets you correct barrel-pincushion distortion on a wide range of camera models. It doesn't seem to allow you to add new models though.
Adding effects is really easy thanks to sliders and you can immediately see the changes in the preview. You can also simply disable an effect you don't want by unselecting it.
The program's interface does feel a little clunky, and you wonder why the directory is on the right, instead of next to the preview. We appreciate that plugins for Photoshop and Aperture and an external editor for Adobe Lightroom come in the installer though, so you can also use it in your favorite photo editor.
The plugins will work exactly as the standalone application.
Despite the slightly unintuitive interface, PTLens is an excellent tool for correcting image distortion.