Netcat: the simplest way to talk to a port

nc -zv host 80

What it does

nc -zv host 80 checks whether a port is open. Run nc -l 4444 on one machine and nc host 4444 on another, and the two machines can send text to each other.

Why it matters

It's called the Swiss Army knife of networking because one small tool does testing, transfers and debugging.

Use it legally

Use it only on machines and networks you own or have permission to test.