website snapshot saved as a png

platform independent tool: 'cutycapt':

CutyCapt is a small cross-platform command-line utility to 
capture WebKit's rendering of a web page into a variety of 
vector and bitmap formats, including SVG, PDF, PS, PNG, JPEG, 
TIFF, GIF, and BMP. 

I'm partial to SnagIt, though it does a lot more than you need.

Other answers have several options for you, but just to throw it into the mix: Your OS probably provides a way to do this.

For instance, on either Windows or Linux (if you're using Gnome or a recent version of KDE) you'd press Alt+PrtScr to copy the active window's image to the clipboard (or just PrtScr to capture the entire desktop, but you said you just wanted the web page). Then paste it into (insert your favorite picture editor here) and save as PNG, optionally cropping out the browser chrome.

This page discusses the screenshot features built into Mac OS X. (Command+Control+Shift+3 copies the entire screen to the clipboard, but you have several other options.)