Very simple little PhantomJS script to render and crop screenshots of webpages.
ghostknife url [left top width height] [pagewidth pageheight] outputfile.png
Render an entire page to a png
ghostknife http://thingsinjars.com thingsinjars.png
Render an entire page in a specific viewport
ghostknife http://thingsinjars.com 640 480 thingsinjars-small.png
Render only a portion of the page
ghostknife http://thingsinjars.com 200 300 100 50 thingsinjars-partial.png