Snopes is your friend.
From Snopes: " We don't know who is responsible for the piece quoted above, but it definitely wasn't acto/comedian Robin Williams (of Mork & Mindy television fame). This item's debut appears to have been a 20 March 2003 posting to the USENET newsgroup alt.motorcycles.harley, and from there it was rapidly disseminated via e-mail and blogs, credited to either "author unknown" or no one at all. The Robin Williams attribution wasn't tacked on until several weeks later, apparently because along the way the eleventh entry was dropped and a genuine Robin Williams quote appended in its place:
"The Statue of Liberty is no longer saying 'Give me your poor, your tired, your huddled masses.' She's got a baseball bat and she's yelling, 'You want a piece of me?'" — Robin Williams.
The 'Robin Williams' attribution for the final item was interpreted as applying to the list as a whole, so now the entire piece circulates as 'the Robin Williams plan.' "