One of the great things about Bethesda is that the developer allows for open modding of its games (sorry 360 and PS3 people, this is for the PC crowd only). This takes your $50 game and turns it into its own little virtual play set.
Now that the game has been out for some months, the range of mods available is getting pretty big. From total conversions to new weapons all the way to custom houses and mini-adventures, there's a gamut of free content just waiting to be sucked through the intertubes straight into your brain. It's almost like your own private Tranquility Lane pod.