I get so tired of people asking, and I have to explain the keyboard shortcuts in OS X to take screenshots. Even though they could (gasp!) look it up in Mac Help or even Google.
Here is a nice article by Dan Frakes that talks about everything you would want to know, from 1st party to 3rd party, freeware to shareware.
Now please, stop asking.
For the quick and dirty methods:
Command+Shift+3: Full-screen screenshotAnd for those who are still using OS 9, you don't get the fancy 3rd option. Upgrade to OS X and you can actually save those screenshots in a real file format to boot.Command+Shift+4: Crosshair to allow selection of screenshot area
Command+Shift+4, then spacebar: “Camera” to capture a specific screen object
