entware allows you to install applications/software that can't be included into Tomato itself, mainly due to the limited flash size availability. For the sake of this example, we'll use a USB flash drive mounted under /mnt/sda1:

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