There are various sitemap extensions , which you would find in Joomla Extensions Directory , but the most famous one is Xmap. Xmap is currently compatible with Joomla 2.5 and 3.0 perfectly and works flawlessly . It has some very cool features , like it can generate XML sitemap and HTML sitemap as well . You can also decide what all menu items should show on your sitemap and exclude what you don't want to show . Another nice feature in Xmap is beautify the XML sitemap page . Also you can have multiple sitemaps created with Xmap.
It's pretty easy to use too. Simply install and create your first sitemap and it will generate the direct links for the XML and HTML sitemaps. You can then simply create a menu Item pointing to your Xmap sitemap. That's it!