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As with the other thread that details how different people will investigate an IP address, i'm interested to hear what free web tools/ sites you use as part of general day to day work.
For example, the SSL test at https://www.ssllabs.com/ssltest/ is really good for scanning a server to see what ciphers are being used, and
MXToolbox (https://mxtoolbox.com) has a variety of really useful tools.
Please share the free tools (this could be applications, but primarily websites/ web apps) that you find useful...