To do a screen shot on MacBook you press and hold Shift+Command+4
Upon doing so you will notice your cursor turns into a cross-hair.
You will click once to start your screen shot and then drag the cursor to the end of the desired image.
BOOM. There ya go!
I love that snipping tool - so easy to use. Take a picture and then copy it and paste wherever you want. I use it every day! Created my slides for a blog post today using it.
On Ubuntu 10.04, go to Applications/Accessories/Take Screenshot or press "print screen" or "PrtScrn"