What are some alternatives?
When comparing phpSitemapNG and XML-Sitemaps.com, you can also consider the following products
DYNO Mapper - Create Sitemaps with the DYNO Mapper Visual Sitemap Generator.
WonderWebWare Sitemap Generator - Sitemap Generator is freeware program that will help you to automatically generate site maps for google, bing and yahoo in their respective file formats.
GlooMaps - Create your visual sitemap on the go - easy peasy
A1 Sitemap Generator - A1 Sitemap Generator has been in active development and sold since 2005.
FlowMapp - UX planning tool for creating a visual sitemap
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.