Firefox
Firefox has an excellent screenshot tool built in. It lets you select the whole tab, draw an area to capture. What's more, it understands the DOM, so it makes it easier to select the DOM component in the web page. This makes it easier to select e.g. a login dialogue or fact box accurately.
Chrome
Currently (2020-11-26) my favourite is 411 screen. It lets you select an area of the web page to screenshot and you can instantly edit the picture, like highlighting the area of interest with a rectangle.
I haven't found a way to copy the image to memory, so you'll have to download it before using it.
Linux command line
Currently (2020-11-26) my favourite is scrot, I typically use it like this:
$ scrot --silent --focused --count --delay 3 -e 'mv $f /home/torstein/pictures/screenshots'
Read the fine manual for more info
