http://gsitecrawler.com/ is a free too that help you generate sitemap for your website that you can submit to Google.
Google Sitemaps allows the webmaster to help Google index their pages optimally. The GSiteCrawler will help you generate the best Google Sitemap file for your website. The GSiteCrawler uses different ways to find all the pages in your website and can generate all sorts of files, statistics and more. The sitemaps file format has lately been also adapted by Yahoo! - even MSN/Live.com is pledging it's support.
But if you have a Dotnetnuke webiste then it already have code to generate sitemap (/sitemap.aspx) and if you have blog then you may want to confogure RSS feed of your blog using FeedBurner and then use that to submit to Google or other search engine.