I've been beta testing url2png.com for a few months now and it's finally been released and opened to public sign ups. The service isn't free, but having the option to avoid the many other screenshot services out there is worth the cost, imo.

Code examples (PHP, Python, BASH, Ruby): https://gist.github.com/1007737

Free DIY alternative that I use: http://cutycapt.sourceforge.net/

Bit more pricy than the other paid alternatives, but perhaps the speed and simplicity is worth it.