Global Redirect is a simple but incredibly useful module for SEO that makes sure users are going to the correct alias of a node since nodes can be aliased in multiple ways (clean url, unclean url, node/###) and duplicate content is a no-no in the SEO world. From the module page:

1. Checks the current URL for an alias and does a 301 redirect to it if it is not being used.
2. Checks the current URL for a trailing slash, removes it if present and repeats check 1 with the new request.

SEO Checklist is a must have Drupal Module for all Drupal sites. It creates a checklist with handy links to the modules/settings that help make your Drupal installation as SEO friendly as possible.
Now I know what you might be saying... "I'm a Drupal expert, I don't need a module to tell me what modules I should be running." Well, I don't believe you. Even the most grizzled developer can forget to add a module or *gasp* not know every Drupal module in existence.