There are a number of Drupal SEO modules to choose from. Here is a summary of the most popular ones to help with your Drupal search engine optimization.
Module Project Page Version Importance Tutorials
Path http://drupal.org/ handbook/modules/path 4/5/6/7 must have Path Tutorial
Pathauto http://drupal.org/ project/pathauto 5/6/7 must have Pathauto Tutorial
Pathauto Settings Tutorial
Page Title http://drupal.org/ project/page_title 5/6/7 must have