This is a service I created for myself to help with generating screen-shots of websites. I can't remember exactly why I needed it initially but it has turned out to be useful on several occasions.
It uses PhantomJS to render an image of a specified webpage at a set of predefined sizes or any custom size. The predefined sizes are 320 x 548, 600 x 1024, 1024 x 768, 1280 x 800 and 1440 x 900.
It's not perfect, the font rendering for example is a little off sometimes and it currently struggles with sites served over https, but most of the time it does a good enough job.