Use osascript
. For example:
osascript -e 'tell app "Finder" to display dialog "Hello World"'
Replacing “Finder” with whatever app you desire. Note if that app is backgrounded, the dialog will appear in the background too. To always show in the foreground, use “System Events” as the app:
osascript -e 'tell app "System Events" to display dialog "Hello World"'
Read more on Mac OS X Hints.