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 handbook/modules/path 4/5/6/7 must have Path Tutorial
Pathauto project/pathauto 5/6/7 must have Pathauto Tutorial
Pathauto Settings Tutorial
Page Title project/page_title 5/6/7 must have

Global Redirect project/globalredirect 4/5/6/7 must have Global Redirect Tutorial
Path Redirect project/path_redirect 5/6 must have
Redirect project/redirect 7 must have
Meta Tags (nodewords) project/nodewords 5/6 must have Meta Tags (nodewords) Tutorial
Metatag project/metatag 7 must have Meta Tags (nodewords) Tutorial
Meta Tags by Path (nodewords bypath) project/nodewords_bypath 5/6 good to have
Meta Tags Node Type (nodewords nodetype) project/nodewords_nodetype 5/6 optional
XML Sitemap project/xmlsitemap 5/6/7 good to have
Menu Attributes project/menu_attributes 6/7 good to have Menu Attributes Nofollow Tutorial
Automatic Node titles project/auto_nodetitle 4/5/6/7 good to have
Search 404 project/search404 4/5/6/7 good to have
Links Checker project/linkchecker 5/6/7 good to have Broken Links Tutorial
SEO Checklist project/seo_checklist 5/6/7 good to have
SEO Friend project/seo_friend 6 good to have SEO Friend Screenshots
HTML Purifier project/htmlpurifier 5/6/7 good to have
Google Analytics project/google_analytics 5/6/7
Keyword Research 6/7
Link Intelligence 6
Content Optimizer 6/7
Scribe SEO Analyzer 6
Drupal SEO Tools 6/7
WordStream Keyword Tools 6/7
Readability Analyzer 6/7
Keyword Analysis 6/7
SEO Essentials 7
Wordtracker 6
W3C Analyzer 6
Service Links project/service_links 5/6/7 optional Service Links Tutorial
Digg This project/diggthis 5/6/7 optional
good to have
Nofollow List project/nofollowlist 5/6/7

SEO Checker project/seo_checker 6/7

Excerpt project/excerpt 5/6

Alinks project/alinks 5/6/7

More Like This project/morelikethis 6

Featured Content project/featured_content 6/7

Related Content project/relatedcontent 5/6

Glossify Internal Links Auto SEO project/glossify 5/6

Calais project/opencalais 6/7

Safe HTML project/safehtml 4/5
Urlify project/urlify 4/5

Find URL Alias project/find_path 4/5
Related Links project/relatedlinks 4/5

URL List project/urllist 6/7
RobotsTxt project/robotstxt 5/6/7
Page Title by Path project/page_title_bypath 5
Custom Breadcrumbs project/custom_breadcrumbs 5/6/7
Pathologic project/pathologic 6/7