Welcome to the forums!
If you haven't already done so, take a look here:
Eyecandy for your GNOME-Desktop - GNOME-Look.org Eyecandy for your KDE-Desktop - KDE-Look.org
Maybe one of those will have something that will work for you.
Not sure, but I was thinking that either xscreensaver, or maybe it was gnome-screensaver had a matrix screensaver with only the letters and digits dropping, but that might be wrong.
Hopefully, someone else might be able to provide a direct link to what you are looking for.