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portchecker.io
portchecker.io is a free online utility to check the port status of a given hostname or IP address.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
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SPF Toolbox
SPFtoolbox is a Javascript and PHP app to look up DNS records such as SPF, MX, Whois, and more
A long time ago, I've built this: https://github.com/frdmn/dnsbl-webutils
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.