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, while not a necessity, can be helpful for larger and dynamic sites to assure all pages are indexed..
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.