From time to time you need to capture a web site in the form of an image. While you can easily use the Ctrl+Alt+PrtSc for windows or Command+Shift+4 in Mac, you might want to screen a site that is tall and or wide and requires scrolling to fully view in your browser. You also might want to perform multiple screen captures of various pages in various web browsers or only grab sections of a page. Below is a list of Windows, OSX and Linux tools you can use to take browser based screenshots in FireFox, Internet Explorer and Safari.
While there are paid services such as Browser Cam that do cross-browser screen captures, I tend to prefer using software since I can fine-tune the results to my liking, and I can see the results immediately.
add-ons via Firefox
ScreenGrab - This FireFox extension installs as an icon in the lower right corner of the he status bar. This plugin uses the Gecko canvas for faster processing. IT allows for capturing either a selection, the visible window or the entire page. You can customize the options to save either a PNG or JPG image and add a timestamp to the filename, and captures frames and flash files sections without any problems.
PageSaver - This alternate extension from Pearl Crescent installs two new entries in the context menu of FireFox, allowing to save either the visibile portion or the entire page, although it does not allow for saving a selection. It has a slew of options however. For example you can assign a keyboard shortcut which is a great time saver. You can also set your image filename patterns and choose the format of the saved image. PageSaver can capture frames and flash files.
Windows Software via Internet Explorer
WebShot - You may want to save the screenshot using IE for testing purposes in which case you need to use WebShot which is a tiny executable written in C by Nathan Moinvaziri. It is available in both a GUI format and a command-line format. WebShot is able to capture any URL and handles frames, and flash files and can also capture java applets. The processing time is somewhat slow for some reason however compared to the other tools. WebShot offers a ton of options when using its command line version.
OSX Software via Safari
webkit2png - is the official command line tool for the Mac which uses python and the WebKit rendering engine, which is used in Safari. Paparazzi - If you need a simpler GUI try Paparazzi which is a great shell on top of webkit2png, making your life much easier. Paparazzi like PageSaver has many similar options you can set for your captured screenshot images.
Unix Software
khtml2png - is a great command line tool for Unix which uses ImageMagick and libkhtml used in Konqueror to create images from web pages.