Certificate search engine. crt.sh is based on the certificate transparency logs and provides wildcard search for domains.

The Common CA Database provides links to general information about CAs and information about their root stores. This covers the root stores of Mozilla, Microsoft, and Google Chrome. Many other useful links are also available, for example, certificate searches and explorers.

DCV Inspector helps you inspect the DNS, HTTP, and SMTP requests made by a certificate authority during domain control validation (DCV). You can use it to detect violations of domain validation rules, such as the use of Delegated Third Parties.

DCV Inspector creates a unique subdomain for each test. All DNS queries, HTTP requests, and emails to this subdomain (including descendants of the subdomain) are recorded and presented to you for inspection.

Google's Transparency Report contains various information. It provides information about email encryption, HTTPS encryption, information about potentially harmful applications in Android, and live reports of traffic disruptions, such as censorship.

It provides a certificate search based on the certificate transparency logs, similar to crt.sh: https://transparencyreport.google.com/https/certificates

Hardenize is a domain hardening checker that covers various aspects. It checks for the presence of basic security extensions and protocols for DNS, email, and web content. This covers such aspects as DNSSEC and CAA records for DNS, email encryption and security like SPF, DMARC, and DANE, and for the web valid certificates and security headers like HSTS or CSP.

The website shows key length recommendations from different agencies, like NIST, NSA or BSI. You can either see the recommendations of one body or compare all for a specific year. The data contains values for symmetric encryption, factoring modulus, elliptic curve, and others.

Website quality measurement tool. The website measures the quality of HTTP headers which improve security. Additionally, it provides inspections for the TLS certificate and SSH servers. It also includes many third-party tools.

Historical certificate dataset. Allows querying based on IP address or certificate.

Test the quality of a server's or a client's SSL/TLS stack. Very useful to test a server. Provides a A-F rating scheme and shows vulnerabilities and weak protocols/cipher suites.

RIPE operates a set of probes, which can be used to send pings or similar measurements. The probes are mainly placed in Europe, but some are also in other continents.

All the collected measurements can be found in the RIPE Atlas Daily Archives. The blog post gives some more details.

The RsaCtfTool is a tool supporting working with RSA keys. The main focus lies in a wide range of known attacks which are implemented and easy to use with it. This makes it suitable for CTFs, especially Jeopardies.

Shodan performs regular scan on common ports.

Access is free, but requires registration. More results can be gained with a paid account.

Test the quality of a client's SSL/TLS stack. The website shows sites which should fail or pass. Bad sites, that should fail but do not, show risks in the client.

Test the quality of a server's TLS stack It shows the enabled TLS versions on the server, tests for available ciphers, checks the TLS handshake, looks at the HTTP security headers, and tests for known vulnerabilities.