Security & TLS
Inspect the live certificate and grade the response headers browsers rely on.
SSL certificate checker
Inspect the live TLS certificate on any host: who issued it, when it expires, which names it covers and which protocol was negotiated.
HTTP header & security check
See the raw HTTP response headers a site sends, and get a graded report on HSTS, CSP, X-Frame-Options and the rest.
Other categories
Reference
Look something up without running a check.
HTTP status codes
What 301, 403, 502 and the rest actually mean, and how to fix them.
Common ports
What listens on each port and whether it should ever face the internet.
Browser errors
ERR_CONNECTION_REFUSED, NXDOMAIN, SSL errors — decoded.
DNS record types
A, MX, TXT, CNAME and friends, with real examples.
Developer utilities
Subnet, cron, base64, JWT, hashes — private, in your browser.