You will be surprised but even without stamp key on the keyboard you can take screenshots whitout 3rd party apps! 😀

There are several keyboard combinations that can be used to take screenshots in Mac OS X. The SystemUIServer process handles these commands.

• Command-Shift-3: take a screenshot of the screen, and save it as a file on the desktop.
• Command-Shift-4, then select an area: take a screenshot of an area and save it as a file on the desktop.
• Command-Shift-4, then space, then click a window: take a screenshot of a window and save it as a file on the desktop.
• Command-Control-Shift-3: take a screenshot of the screen, and save it to the clipboard.
• Command-Control-Shift-4, then select an area: take a screenshot of an area and save it to the clipboard.
• Command-Control-Shift-4, then space, then click a window: take a screenshot of a window and save it to the clipboar.

From Mac OS X Leopard and later, the following keys can be held down while selecting an area (via Command-Shift-4 or Command-Control-Shift-4):

• Space: to lock the size of the selected region and instead move it when the mouse moves.
• Shift: to resize only one edge of the selected region.
• Option: to resize the selected region with its center as the anchor point.

NOTE: different versions of Mac OS X have different formats for screenshots.

NOTE2: in Mac OS X 10.4 and later, the default screenshot format can be changed, by opening Terminal app and typing :
defaults write com.apple.screencapture type image_format
killall SystemUIServer
Where image_format is one of jpg, tiff, pdf, png, bmp or pict (among others). If you omit the second line, you will need to log out and in again for the change to take effect.