How to add vignetting?

I only know how to do it in PS. Just open the image in PS then go to Filters > Distort > Lens Correction. Then you will see a slider on the right for vignetting.

Happy Vignetting.

Alternatively, if you're using raw files from your camera, Adobe Camera Raw has a vignetting add/remove feature under Lens Corrections (at least in CS2 and CS3; That's ACR3 and ACR4)

I've never seen that availabe in Aperture. It is of course possible in CS3

CS3....and it's incredibly easy in Lightroom.

you can even do it in iPhoto

In Lightroom is pretty much consists of two sliders which you drag along to get whatever effect you want.