If you use the Command+Shift+3 or Command+Shift+4 commands to save screen captures, you know how quickly your desktop gets cluttered with mysteriously named PNG files. A post on the Sneak blog points to a solution to this problem for Leopard users:
To change the default save location, open up Terminal and run the following command:
defaults write com.apple.screencapture location /Users/YOURUSERNAME/Documents/Screenshots
Your screengrabs will be saved to the Screenshots folder on your desktop, which you can drag onto the dock and use as a Stack.
Unfortunately, this doesn’t work for those of us who still run Panther. If you’re a Pather user, your best alternative is just to use Command+Shift+Control+3/4. The image will be saved to your clipboard instead of the desktop, which you can then paste it into Photoshop. A File->New in Photoshop will default to the height and width of the clipboard’s contents.