just go to joomla.org up top click on extensions then first on left is admin tools just click on search engine friendly URLS
here is great list Smart Extensions To Optimize Joomla for Search Engines Ranking Higher http://visionwidget.com/inspiration/web/430-joomla-optimize-earch-engines-extensions.html
http://joomboss.com - SeoBoss is a Joomla extension which allows to configure site to be best indexed and understanded by Search Engines.
if you only to user Joomla com_contents as default, I think Joomla SEF core is good and if you to use K2, turn on K2 SEO with joomla core SEF will better than use 3 rd extensions goodluck