If you are looking for a tool to create website screenshots have a look at the Firefox extension FireShot.

Besides taking screenshots from the visible part of a web page it can also create images of the complete page, which is what I was looking for.

I stumbled across this extension in Stuart Herberts blog post about 40 Firefox extension for web developers, which contains many interesting extensions – some of them have been mentioned here earlier.

UPDATE: Unfortunately, FireShot is not available for Mac OS X.

UPDATE 2: Looking for a solution running also on the Mac I found Screengrab!