Global Redirect

Global Redirect essentially makes sure that each page on your site has one and only one URL that points to it. For instance, by default Drupal will accept a URL with our without a trailing slash; both URLs will go to the same page. This is potentially bad for SEO as search engines might penalize your site for having "duplicate content" (the same content at more than one address). Global Redirect will strip those trailing slashes, as well as perform a handful of other operations with the same goal. You can read the specifics of it on the project page.