Free XML Sitemap Generator
Sitemaps help the Search Engines determine the correct pages to spider and how often the pages of the site are updated. All Search Engines use a format the lets them read XML files and aids in your site being properly indexed. Having an XML sitemap can be helpful for larger and dynamic sites to assure all pages and media are indexed by Search Engines.
To dispel any myths: Sitemaps do NOT help sites rank better. Their purpose is to assure that all website pages are accessible to the Search Engines.