Ping – test connectivity to device – built in.
Nslookup – A command-line program that queries domain name servers to find information about a domain or IP address – built in
Wireshark – let’s you analyze packets https://www.wireshark.org/download.html
URLVoid – Checks the reputation of a website. – https://www.urlvoid.com/
URLscan – Let’s you scan a website to see what the website using/doing. urlscan.io
Virustotal – Checks the reputation of a file, ip address, or URL . For files, you will need the hash. https://www.virustotal.com/gui/home/upload
IPvoid – Gives WHOIS information and check ip reputation. ipvoid.com
Any.run – Free sandbox to check if a file is malicious. https://any.run/
Anything by Eric Zimmerman – Eric Zimmerman is a legend in the community and wrote a ton of free tools. https://ericzimmerman.github.io/#!index.md