If you need a good way to take screenshots and maybe note something on it, circle something, then Shutter is the perfect application. It takes screenshots and you can edit them straight away.
Install it using Ubuntu Software Center or write the following in a terminal.
sudo apt-get install shutter
You can take screenshot of the whole screen, just a program or a selection of the screen. It also contain a light way editor to add text, circles, arrows, hightlighting and other stuff thats very useful when sending someone a screenshot. I most often take a selection and then add circles and text to it. I use it for documentation/manuals and explaining stuff to coworkers and customers.
I’ve been using it on Ubuntu 11.04, Ubuntu 11.10 and Xubuntu 11.10 so far.