A selection of must have Joomla Freeware Components:

sh404SEF Component
This is for User and Search Engine Friendly Urls more like metatags improvements and proper 404 error coding..
It has a lot of features, just make sure your .htaccess file is set for Third party Sef Components (read the comments in the htaccess.txt file)
http://joomlacode.org/gf/project/sh404sef/frs/

Alhpa Content
The Alpha Content component is a hidden treasure when it comes to Search Engine Optimization.

Description Intro form the website:
AlphaContent is a directory of Yahoo style with an alphabetical research for item. This directory is built with a tree structure similar to yahoo and the other major directories starting from the sections and categories of Mambo/Joomla.
 

And this directory structure is really great on bringing food to search engine spiders!!
http://www.visualclinic.fr/component/option,com_remository/Itemid,39/func,select/id,3/

Joomla SEF Patch
This Patch in the free version is not a component, but a simple FTP upload and overwrite improvement that will gie you some great features.
- Meta descriptions and keywords for menu items.
- Fix for Keyword stuffing (repeat) if you have filed the Global Meta tag field
- Remove the Joomla version information in the source code.

You can find the right patch for your Joomla version at:
http://www.joomlatwork.com/downloads