Here are two options.
Webkit2png (free for Mac) - http://www.paulhammond.org/webkit2png/
Khtml2png (free for Linux) - http://khtml2png.sourceforge.net/
Paid online api - http://url2png.com/
Now that I've set up phantomjs (headless webkit) on a VPS per petewarden's suggestion, I can see what it costs to run a screenshot farm myself. A couple pennies per thousand images at most for bandwidth -- Linode's smallest instance can run a bunch of webkit instances in parallel just fine. I've already generated and resized almost 1000 screenshots since installing it all.