Web Services for Analyzing Website Information via URL

I will list only the tools that are useful for things like competitor research.
I have omitted services primarily used for checking your own site, such as SSL health checks, as there would be no end to the list.

SimilarWeb

https://www.similarweb.com/ja/
Estimates of traffic volume and access trends.

aguse.jp

https://www.aguse.jp/
General server information.

Netcraft

https://toolbar.netcraft.com/
Also general server information.

Wappalyzer

https://www.wappalyzer.com/
Technology stack investigation.

BuiltWith

https://builtwith.com/
Also technology stack investigation.

Rayst

https://ray.st/ja
Also technology stack investigation.

DNSDumpster.com

https://dnsdumpster.com/
Infrastructure-related.

Web Check

https://web-check.xyz/
Various information.

urlscan.io

https://urlscan.io/
Various information.

Virus Checks

Safe Browsing: Malware and Phishing – Google Transparency Report
https://transparencyreport.google.com/safe-browsing/search

VirusTotal
https://www.virustotal.com/

urlQuery.net
http://urlquery.net/

List of things intentionally excluded

• Email address blacklist check systems (Probably already summarized in many places)
• SEO systems (Same as above)
• Speed test systems (Same as above)
• Whois systems (The features are so similar everywhere that I just use whois.com or who.is; please let me know if you have recommendations. Update: Recently I've been using https://domainwat.ch/)
• ahrefs (Because it's paid)
• Alexa (Now synonymous with home speakers)
• SimilarSites (Since you can see them on SimilarWeb...)

See Also

Investigating the technologies used on that site - Qiita
https://qiita.com/nightyknite/items/1258eeca403e7d449b5a