Sitemaps
Yes, it’s a good thing to have on your site and be sure to consider it in any new design plan, or updating a current site. The site should have an up to date XML sitemap (per Google), as well as an HTML version that will show the entire site structure in one place for easier spidering (as well as helping out a lost visitor). Yahoo has requested a text version of the sitemap to be submitted, as well.
Recently, however, a new procedure has been adapted in an effort to make the sitemap process easier. By adding a line on the robots.txt file that all sites should have uploaded, the spiders are alerted to the location of the sitemap and will immediately go to it. It is my understanding that Google is currently recommending the continuation of submission to the Google Sitemaps site along with the Robots.txt file inclusion.
It’s a simple line of code that looks like:
Sitemap: http://www.mysite.com/sitemap.xml
Read more about Google’s interpretation of this new procedure here: Sitemaps as part of the Robots.txt file