How do I find the windowid to pass to screencapture -l?
For some applications you can use AppleScript:

screencapture -l$(osascript -e 'tell app "Safari" to id of window 1') test.png


It doesn't work with Chrome though.

The IDs are also shown in Quartz Debug (available from developer.apple.com/downloads) if you run defaults write com.apple.QuartzDebug QuartzDebugPrivateInterface -bool true.