As a web design and marketing company, we often have to take full length web screenshots. Shooting one screen at a time and putting it all together in an image editor like Photoshop afterwards is so 2000.
If you do want to do it the hard way, you just need to use the built-in shift-command 3 (whole screen) or shift-command 4 (selection) and grab the images from your desktop.
Here are the various tools we use to make full length website screenshots easier:
Layers is a really powerful screenshot tool which shoots your whole screen but in its attendant layers, allowing you to isolate elements. It's fantastic for an OS or application developer building documentation or an application designer creating prototype interfaces and has a great function for web designers, Web shots which will shoot any Safari or Webkit browser window from head to toe. Afterwards you do have to crop off the browser chrome but Layers works consistently and allows you to screenshot password protected and https pages (as it shoots the window and not the URL).
I'm not sure how well Layers works on 10.7 Lion and 10.8 Mountain Lion. Performance in Snow Leopard is acceptable, although it occasionally does choke and misbehave.
Netfixer is just a plugin the URL and go tool. Works great and fast. Totally reliable on Snow Leoapard 10.6.8. I'm not sure why our friends at Shiny Frog abandoned Netfixer. Netfixer never took off in popularity. Probably because of the very silly name. I warned them about that. We have a brilliant front ends comment manager for WordPress called Thoughtful Comments which never took off either despite the occasional rave review asking the same question. Not sure if Netfixer works in later builds. The open source Netfixer repository is here. Here's a link to a zip of a .2 build from 2006 which still works fine on Snow Leopard.
Netfixer is not enough for us as we often want to shoot https pages which fail.
At last a website screenshot utility with a decent name. I've always wanted to like Paparazzi on the name alone. The original icon was quite sexy too, even if it is not high enough res for modern versions of OS X. Unfortunately Paparazzi! has failed quite a bit depending on your OS version. The current 0.6.7, the first public release since 0.4.3 (I smell private internal revisions/fixes which never went out to the public: not very nice), works great on 10.6.8 and allows you to screenshot to the width you want. This is really useful for a web designer to give examples to a client of the effect of a design on different devices.
Paparazzi! also has preferences to allow click-to-flash, choose custom stylesheets, choose user-agent and to accept all https certificates (superb!) by default. There's even some preferences for batch screenshots (not sure why someone would want screenshots of a whole website, but it's nice that derail cares enough to offer them: I guess for law enforcement or legal reasons batch might come in handy).
The new Paparazzi! works great and it's the full length website screenshot tool we recommend. 0.6.7 is so fresh that there are no development notes yet but hopefully they will come soon.
Note: We know there are some full length screenshot tools for Firefox and may be for Chromium too. We don't use the Firefox versions as we have enough developer extensions enabled on Firefox, that often the display would end up being non-standard anyway (some little SEO colours or flags somewhere or Flash disable). Getting Firefox into condition for proper screenshots would be five or ten minutes/pop depending how often one does it. And even then one would have a trace of doubt if the screenshot was accurate or not.