The Synaptics Package Manager is a GUI-based application which is useful for adding and removing packages to/from your system. The Ubuntu Software Center is not. Many users who upgrade to a later version of Mythbuntu, from an earlier version, will miss the Synaptics Package Manager. The won't find the selling of magazine subscriptions by their package manager, while they are trying to install software, to be an amusing feature. Fortunately, you can easily add the Synaptics Package Manager back in, from the command line, like this:
sudo apt-get install synaptic
The "Useless and Annoying Software" section details how the Ubuntu Software Center package can be removed, if you don't wish to keep it around (and, why would you).
Using the Synapics Package Manager, you can install all of the useful packages you'll ever need. Here is a list of some of the one's that we've found useful:
emacs
vsftpd
sendmail (if you'd like email delivery)
mailutils (mail utilities such as "mail")
You can also install these packages using apt-get, as follows:
sudo apt-get install emacs
sudo apt-get install vsftpd
sudo apt-get install sendmail
sudo apt-get install mailutils