Entware is a software repository for network attached storage, routers and other embedded devices. 2500+ packages are available for different platforms. The packages are full versions found in common Linux distributions. You can expand on the BusyBox versions of commands included with FreshTomato and install packages not installed to “push the limits” of your router (figuratively of course). See how to use it.

You can find current, in-depth instructions for installing Entware on FreshTomato on Entware's wiki.
Entware on FreshTomato (and other TomatoUSB forks)

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, please see the USB Support page.

First, create the opt folder and mount it:

mkdir /mnt/sda1/opt
mount -o bind /mnt/sda1/opt /opt

Then, you can run the installation command:

/usr/sbin/entware-install.sh

Finally, you can install the packages you want, for example:

opkg update
opkg upgrade
opkg install htop