How do you get the effect on pics where the edges are darker?

[SAWYER]

http://www.flickr.com/photos/leoville/4399901930/

Like that pic

 

[Anubis]

i cant see the pic (blocked at work) but im gonna assume you mean this

http://en.wikipedia.org/wiki/Vignetting

 

[timswim78]

i cant see the pic (blocked at work) but im gonna assume you mean this

http://en.wikipedia.org/wiki/Vignetting

Yes, he is talking about vignetting.

OP, many editors, such as lightroom, have that effect built in. It happens naturally on some lenses when they do not allow enough light in at the edges of the frame.

 

[Aharami]

its called a vignette. In photoshop you can either go to filters> distort> lens correction, and add a vignette that way, or this is how I do it.
select the lasso tool. set feather to something like 175 pixels. draw a rough rectangle with rounded corners (or any other shape you want) around 5-10% in from the picture frame. select inverse. adjust levels to make selection darker. This way you can give it a custom vignette instead of just darkening the corners.