Star Wars: Knights of the Old Republic is an PC/Xbox CRPG game originally released in 2003, developed by Bioware.
It takes place in the Star Wars universe (yes, the same as the movies!), and uses the d20 roleplaying system with turn-based combat and has various included mini-games, such as Swoop racing and a card came called Pazaak.
The game is very rich with dialogue and side-quests, making the storyline seemingly less linear. You are accompanied by several NPCs with their own personalities who joins your quest and provide their thoughts on the situations you encounter, as well as give support in combat. You can keep up to two NPCs with you at all times.
There is a light path and a dark path in the game - your actions have consequences and you can choose to either follow the good or turn to the dark side. Beware, though; your companions might not like the choices you make!
The game does not have an expansion, but it does have a sequel - Star Wars: Knights of the Old Republic II: The Sith Lords.
Compared to the Star Wars movies, the game is set about 4000 years prior to them. As the game begins, the battleship you are on is attacked by your enemy; the Sith, led by the vicious Sith Lord Darth Malak. You and one of your companions manage to flee the ship to land on the planet Taris. Now, you have to find another person who had been on the ship, the jedi Bastila Shan. Together, you can flee the planet and seek refuge in the jedi enclave on Dantooine.
From here on, you travel to various planets in the galaxy, to search for the source of the enemy's power; the Star Forge. Can you find it in time, before Darth Malak's armies grow too powerful? Can you defeat the Sith Lord before he conquers? The fate of the galaxy is in your hands...
Various fan created tools can be used to mod the game - check here for a list of popular and useful modding tools.