With my recent discovery of Adobe Photoshop Lightroom 3, I have found the vignette tool to be very helpful in post-capture processing to give a professional and creative look.
The process of a vignette was originally (and still can be) and unintended effect of the lens due to the way it curves outwards allowing less light around the edges of the capture. As you can see from this example image, vignetting allows the image to have greater focus in the centre by darkening the edges.
|With Vignette (and other processing)|
|Without Vignette - Original Photo as Shot|
The simplest way to apply this vignette is using Lightroom (which is what I used above as well as for my other images). There are three options, but the default "Hightlight Priority" is the best one to use as this ensures minimal loss of detail around the edges, where it does exist.
This below video gives a good introduction and example of how to apply this effect properly (using Lightroom) to your images. I found this very useful, recommend you watch it!
Slightly more difficult, but probably more commonly used is the Lens Correction Filter in Photoshop. For a guide on how to do this in Photoshop, click here.