We at DrupalChamp care about SEO. We will put Drupal SEO modules at no cost  to you. These SEO modules are very important to make a site SEO enabled site. we found following SEO modules very useful.

path (Drupal 7.x, 6.x, 5.x)
A core module that lets to manually specify human-friendly and search engine-friendly urls.

pathauto (Drupal 7.x, 6.x, 5.x)
Automatically generates url path aliases that are human-friendly and search engine-friendly.

path redirect (Drupal 7.x, 6.x, 5.x)
Redirect from one path to another path or an external URL, using any HTTP redirect status.

global redirect (Drupal 6.x, 5.x)
Eliminates duplicate content that is harmful to your pagerank by properly redirecting alias urls. also fixes trailing slashes.

pathologic (Drupal 6.x)
An input filter which can correct paths in links and images in your content in situations which would otherwise cause them to ?break?.

search 404 (Drupal 7.x, 6.x, 5.x)
When an URL generates a 404 file cannot be found error, this module performs a search on the keywords in the URL and displays the search results.

page title (Drupal 6.x, 5.x)
Gives granular control over the page title. specify token patterns for how the title should be structured. Specify custom page title that is different from node title.

nodewords (Drupal 6.x, 5.x)
Set meta tags for each node, view, or panels page.

meta tags by nodetype (Drupal 6.x, 5.x)
Extends the functionality of nodewords so that meta tags can be specified on a per node type basis.

meta tags by path (Drupal 6.x, 5.x)
Extends the functionality of nodewords so that meta tags can be specified on a per path basis by using user-defined rules.

custom breadcrumbs (Drupal 6.x, 5.x)
Displays keyword-rich anchor tags on your site that are both search engine friendly and user friendly.

menu attributes (Drupal 6.x)/
Specify additional attributes for menu items such as id, name, class, style, and rel. Useful for sculpting pagerank flow through your site with rel="nofollow".

service links (Drupal 6.x, 5.x)
Easily adds links to a number of social networking sites for your nodes. this will help promote your pages and get valuable backlinks to increase your pagerank potential.

digg this (Drupal 6.x, 5.x)
Adds a Digg this button to your nodes. Useful to increase the popularity of your pages and boost your pagerank potential.

tweet (Drupal 6.x, 5.x)
Provides links for your visitors to post urls to twitter. Useful to increase the popularity of your pages and improve your pagerank potential.

twitter (Drupal 6.x, 5.x)
Associate users with their twitter accounts. post tweets anytime new content is created on your site. Promotes your sites and increases your pagerank potential.

ping.fm (Drupal 6.x)
Integration with Ping.fm which makes sharing posts on other social networks easy. Helps the popularity and pagerank potential of your site.

add to any (Drupal 6.x, 5.x)
Helps readers share, save, bookmark, and email your pages to social networking sites. Improves the popularity and pagerank potential of your site.

xml sitemap (Drupal 7.x, 6.x, 5.x)
Creates a sitemap.xml file that conforms to the sitemaps.org specification. Helps search engines to intelligently crawl your website.

robotstxt (Drupal 7.x, 6.x, 5.x)
Edit robots.txt from the Drupal admin ui, generates the robots.txt file dynamically, multi-site compatible.

google analytics (Drupal 7.x, 6.x, 5.x)
Integrates google analytics web statistics with your website.

seo friend (Drupal 6.x)
Compliments existing SEO compliance modules and provides several reports to show recommended meta tag and title changes.

seo checklist (Drupal 6.x, 5.x)
It provides a checklist that helps you keep track of what needs to be done.

seo checker (Drupal 7.x, 6.x)
Checks node content on search engine optimization upon its creation or modification.