SimilarWeb - SimilarWeb.com is a website analysis tool that gives you analytics information for any website.
Enlight - Performance and Error Monitoring. We keep an eye on your applications and notify you about performance issues and errors.
Website Informer - Website.informer: complete data lookup & free aggregated report on any domain including whois, visitors, IP & DNS details, competitors, owners, etc
Lambda School - A full Computer Science education - free until you get a job
Site Rank Data - View historical rank information of websites via time series charts.
Free Code Camp - Learn to code by helping nonprofits.