Screen Capture has also been improved big time in Jaguar. When you use the interactive version (command-shift-4), the space bar will toggle between the regular marquee selection mode and a new mode to capture just a single window, the Dock or the menu bar. The cursor changes to a camera icon when you are in this new mode and as you mouse over different areas of the screen the screen capture target is highlighted for you.

And of course, there’s more… Screen captures can be triggered from the command line (and therefore AppleScript, perl, etc.) using the new screencapture command found in /usr/sbin/

usage: screencapture [-icmwsWx] [file] [cursor]
-i capture screen interactively, by selection or window
control key – causes screen shot to go to clipboard
space key – toggle between mouse selection and
window selection modes
escape key – cancels interactive screen shot
-c force screen capture to go to the clipboard
-m only capture the main monitor, undefined if -i is set
-w only allow window selection mode
-s only allow mouse selection mode
-W start interaction in window selection mode
-x do not play sounds
file where to save the screen capture

—> thanks to Ken Bereskin for this info