The global redirect module is a must have for SEO on a Drupal site.  It does a great job helping with drupals "duplicate content" issues and redirecting urls around your drupal install. 

For example by default, drupal shows the same content for a both a url ending in a slash "/" or not ending with a slash:

• mysite.com/node/36/
• mysite.com/node/36

To drupal this is the same page, but to a search engine, these might be very different pages.

My global redirect settings:

Here are the settings I've found to work well with the search engines.

• Deslash: ON
• Non-clean to Clean: ON
• Remove Trailing Zero Argument: Enabled for taxonomy term pages only
  • I've only really had issues w/ trailing zero's showing up in search engines with taxonomy term/vocab pages.  That is why I only turn this on.
    
• Menu Access Checking: Dissabled
• Case Sensitive URL Checking: Enabled

Global redirect module along with a good robots.txt file will help you get rid of alot of the duplicate content on your site that may hurt your SERP rankings.