Although lacking in story, and considered non-canonical in the Fallout universe, Fallout Tactics is an excellent game that can easily stand on its own.
Departing from its predecessors in a number of ways, Tactics is most notable for its well-rounded and exceptionally balanced multiplayer component. Using a holistic point-based system, the Multiplayer character creator allows you to choose your character's level, skills, perks, and equipment. As you go, it keeps a running point tally, denoting the character's overall value. When you join a multiplayer match, the host sets a maximum character point value, to keep gameplay balanced, and you can only use characters with a total value equal to or less than the maximum.
This mechanism alone allows massive customization and flexibility in building a team. It also allows you to vary the type of gameplay from the skirmishes between low-level, poorly-equipped wastelanders to the clash of chain-gun wielded, power-armored gods.
Buy two copies of this game; one for you and one for your best LAN buddy, and you will both be hooked.