What are some alternatives?
When comparing PerfectPage and phpSitemapNG, you can also consider the following products
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
DYNO Mapper - Create Sitemaps with the DYNO Mapper Visual Sitemap Generator.
UptimeRobot - Free Website Uptime Monitoring
XML-Sitemaps.com - Free Online Google Sitemap Generator.
StatusCake - StatusCake | Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.