I have seen this question being asked a number of times in the forums and thought I'd post about it.
What is a Google Sitemap, who better to answer but Google:
A Sitemap provides an additional view into your site (just as your home page and HTML site map do). This program does not replace our normal methods of crawling the web. Google still searches and indexes your sites the same way it has done in the past whether or not you use this program. Sites are never penalized for using this service. We cannot make any predictions or guarantees about when or if your URLs will be crawled or added to our index. Over time, we expect both coverage and time-to-index to improve as we refine our processes and better understand webmasters' needs.
In CS 2007, Google site map indexing is built-in and you don't need a module. You can find this at [your-blog-url]/GoogleSiteMapIndex.ashx. E.g. for this blog the Site map index can be found here.
You of course need to submit a Sitemap file to Google and have it validate your domain.