Google, Microsoft and Yahoo have announced a new tag <link rel="canonical">, which once added into the head section, tells the search engines about your preferred canonical URL it should have for the current page. For e.g. you specify the following URL in the <head> section:
<link rel="canonical" href="http://www.mysite.com" />
Now search engines will tread the above URL as preferred and the following URLs as duplicate contents, and will avoid crowling & indexing:
For WordPress.org, Magento, and Drupal users, Joost de Valk has created a new canonical URL links Plugin, you download here.
More info: Google Webmaster Central Blog