How to find which process is using a specific port?

$ lsof -it tcp:80

Install lsof

$ sudo apt-get install lsof     # Debian, Ubuntu
$ sudo yum install lsof         # RHEL, Fedora, CentOS
$ sudo pacman -S lsof           # Arch
$ sudo zypper install lsof      # OpenSUSE
$ sudo emerge -a sys-apps/lsof  # Gentoo