I have recently found this free sitemap generator and think it is one of the best. http://www.vigos.com/products/gsitemap/ You can download it from here: http://www.vigos.com/download.php?product=gsitemap What do you think?
www.autositemap.com is the latest I have used although I do not think they allow new members at the moment. I removed my sitemaps the other day to see what happened
Sitemap services that create sitemaps without errors are great, but not all of them can catch errors on your site that may stop spiders once they get there. I hear a lot of people talking about having sitemaps and still not getting indexed. Just because Google has your URL doesn't mean that they'll list it or fully index it. That's a quality issue on your page that most sitemap services can't catch or report to you, but some can....
There are also various other factors in my opinion Can it report 404s (etc.) and show from a site was linked and/or redirected from. Can is calculate priority etc. for Google XML sitemaps. For HTML sitemaps, the amount of options / templates. Can the sitemap crawler perform logins (can be relevant for HTML sitemaps). Can it be automated. Does it support website crawler filters. Can it cut session ids in urls if need to be (usually not necessary if enabling cookies support).