A while after  Fallout 3 came, Bethesda started coming out with add-ons to expand the game even more.

They first created the The Pitt.  This one features a new city.  One of the most important things to the storyline in this add-on are Trogs.

Then they made Operation: Anchorage.  In this one you enter a virtual reality, which also results in an all-new world.

Next came Broken Steel.  This Add-On starts once you complete Fallout 3.  In this one you help destroy what's left of the Enclave.  You also can continue playing the game after it is complete even when you complete Broken Steel's main quest.

The fourth one was Point Lookout.  You go on a boat to travel here.  The main villain is the Brain.

The fifth and last one is Mothership Zeta.  The Crashed Alien Ship Random Encounter is a key part to this add-on.  You go aboard an Alian Mothership, and must fight your way out.