Entware is a software package installer. It allows you to install programs that can't be included into Tomato itself, mainly due to most routers' limited flash RAM size. This example assumes you have a USB flash drive mounted under /mnt/sda1. If you need more details about configuring USB storage before reading this, see the USB Support page.
First, create the opt folder and mount it:
mkdir /mnt/sda1/opt mount -o bind /mnt/sda1/opt /opt
Now you can run the installation command appropriate for your build:
ARM: wget -O - http://pkg.entware.net/binaries/armv7/installer/entware_install.sh | sh MIPS: wget -O - http://pkg.entware.net/binaries/mipsel/installer/installer.sh | sh
Finally, you can install the packages you want e.g.
opkg install htop