What about AutoIT http://www.autoitscript.com/autoit3/ (on windows platforms)?
Similar tools probably exist for other platforms...
I realize that platform-independent tools are preferable, but at the same time, platform-specific tools can leverage the underlying windowing API to know what to poke (getting coordinates of controls, simulating keyboard input, simulating mouse clicks), and poke it in a way that is unlikely to be any different from the way a user does it.
Such tools could also be used to check for race conditions exposed by rapid user interaction (cases such as the infamous Therac example).