There are a lot of free SEO tools for executing simple SEO activities, e.g. building simple sitemaps, optimizing layered navigation, running basic SEO site audit, etc. You may find all of them on Magento Connect: https://community.magento.com
As you can see, the most popular SEO plugin is the one by Creare.
But note that some of the free Magento SEO modules may be of a low quality and do not work as expected. So we recommend you to read the reviews and check the demos first.