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