What are some alternatives?
When comparing phpSitemapNG and Netpeak Spider, you can also consider the following products
DYNO Mapper - Create Sitemaps with the DYNO Mapper Visual Sitemap Generator.
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
XML-Sitemaps.com - Free Online Google Sitemap Generator.
SEOmator - SEO website analysis & audit tool
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.
Screaming Frog - Screaming Frog is search engine marketing agency offering search engine optimisation and pay-per-click advertising services.