It is often advised to have a sitemap to not just help your visitors but also to help the spiders crawl all the pages easily. While it is true that for smaller websites with a good HTML based navigation a sitemap might not be required more often than not site's face indexing issues because of improper navigation structure. Also for bigger websites with huge number of pages there is a possibility that the search engine bots might not be able to find all the pages by themselves and unless your pages get indexed properly there is no chance of getting search engine rankings.
Keeping all these in mind it is always better to have a sitemap or may be two - one HTML sitemap for your visitors and for search engines and one XML sitemap exclusively for the spiders.
For those of you who might be stumped with the XML bit and not know how to write a XML sitemap there are a bunch of tools available on the web that allows you to create a XML sitemap in a ziffy.
Here is a list of such Sitemap generator tools that you might find useful.
- XML Sitemap Generator ( by far the most popular tool )
- Google Sitemap Generator Beta
- Google Sitemap Generator software for Windows
- Google's Guideline's for Sitemap Creation
- HTML and XML sitemap Generator tool
Also sitemap's can be very effective if you would like to get other forms of content indexed by the search engines. There are specific type of sitemaps like video sitemap, code search sitemap, News sitemap etc can be very handy.