IMHO Screengrab is the best screenshot-addon for browsers i know, because it can be highly configured:

• you can save the whole site, the visible area or a selection
• the image-format can be chosen between PNG, JPG or BMP
• the automatic name of the created file can be configured
• the functions behind the icon can be configured
• hotkeys can be set
• upload-services can be pre-configured
• is translated into a bunch of languages
• is open source (MPL2) - at least the firefox-version, no idea about the chrom(ium)-version
• and it is available for firefox and chrom(ium)

Screengrab is the best screenshot add-on for browsers I know, because it can be highly configured:

• you can save the whole page, the visible area or a selection
• the image format can be chosen between PNG, JPG or BMP
• the automatic name of the created file can be configured
• the functions behind the icon can be configured
• hotkeys can be set
• upload services can be pre-configured

It is:

• translated into a bunch of languages
• open source (MPL2) - at least the Firefox version, no idea about the Chrome/Chromium version
• available for Firefox and for Chrome/Chromium