top  // frequently-updated list of processes
http://en.wikipedia.org/wiki/Top_(Unix)

netstat -a // network connections (both incoming and outgoing)
netstat -a | grep 3306 | wc -l   // (how many connections on port 3306)
http://en.wikipedia.org/wiki/Netstat

ps  //currently-running processes
http://en.wikipedia.org/wiki/Ps_(Unix)

uptime //time a computer system has been "up" and running
http://en.wikipedia.org/wiki/Uptime   

sar // activity for the CPU
http://en.wikipedia.org/wiki/Sar_in_UNIX

free // free memory
free -m  

who // whos there ?
http://en.wikipedia.org/wiki/Who_(Unix)

uname // prints the name, version and other details about the current machine and the operating system running on it
http://en.wikipedia.org/wiki/Uname

Some of  my most useful commands. For more help use the <UNIX-COMMAND> --help

Useful Unix commands for managing a server

Bash