Applescript

tell application "Scriptable Text Editor"
        make new window
        activate
        set contents of window 1 to "Hello World!" & return
end tell

submitted by: eric@nymt.reuter.com (Eric Goebelbecker)

Here is an AppleScript "Hello World!" which repeats endlessly:

tell application "Scriptable Text Editor"
  make new document
  repeat
    make text at end of front document with data ("Hello World!" & return)
  end repeat
end tell

And here's one that doesn't rely on an external application:

on helloWorld()
  display dialog "Hello World!" buttons {"OK"}
end helloWorld

helloWorld()

submitted by: stevep@wrq.com (Steve Poole)