What is the program that suggests installable packages?

command-not-found is the package you are looking for.

Just as an addition to the previous answer:

To switch this off for your own account (without deinstalling the whole package and so disabling this for ALL users), you may just add the following line to your own ~/.bashrc:

unset command_not_found_handle

This reverts to the default bevaviour of bash (see /etc/bash.bashrc for the definition of the function command_not_found_handle: it calls command-not-found if it is installed, otherwise it prints the default "command not found" error message).