On Dantooine during your Jedi training you will be sent to investigate someDark power emanations in the wilderness. When you find Juhani, you can turn her to the Light Side and have her join your party, or you can threaten to kill her. Do so and you will set off an interesting chain of events that not only lose you Juhani as a party member (since she's dead) but sends another Jedi member towards the Dark Side.
Easy Heal
Anytime you are in a safe area (not in combat or inside a room) you can instantly heal yourself. Go to the Map screen in the pause menu and hit the X Button to instantly leap back to the Ebon Hawk. Your party members will be magically healed. You can then select the Map screen and hit X again to jump back to where you just were.
Kill Good Guys on Tatooine
After helping the men defend the Sand Crawler in the desert, put on Sand People outfits (taken from dead Sand Folk) and return to the Sand Crawler. The men will attack you, thinking you are enemies. You are free to defend yourself and kill them. Truly an evil act.
Earn Different Endings
Finish game on the Light Side (by doing good deeds and denying the Dark Side) to see the Light Side ending. Finish game on the Dark Side (by doing evil and accepting the Dark path) to see the Dark Side ending.
Extra Sidequests
Talk to the other members of your party often. Most have an extra side quest that can only be activated by talking to them at least 8 times during the game. Sometimes it takes even more. Bastila's, for example, has you searching for her mother on Tatooine and Carth's has you hunting for his lost son.
Finding Party Members
Bastila - Jedi Human - Taris
Canderous - Mandalorian Soldier - Taris
Carth - Human Soldier - Endar Spire
HK-47 - Combat Droid - Tatooine (buy him in the droid shop)
Jolee Bindo - Jedi Human - Kashyyyk
Juhani - Jedi Guardian - Dantooine (don't kill her!)
Mission - Tw'lek Scoundrel - Taris
Zaalbar - Wookie Scout - Taris
T3-M4 - Expert Droid - Taris
Get extra items
On the Ebon Hawk, talk to each member of your crew. You can ask each one to make you specific items:
On Korriban in the Valley Of The Dark Lords there are four tombs. One of these tombs is full of droids that attack you. Destroy all of the hostile droids and open the door at the end of the long straight corridor. There is a special droid that talks to you and tells you that he wants to leave the Sith world because he doesn't agree with killing people. He then asks you to help him by shutting him down.
This will give you a lot of light points and a lot of experience.
Infinite Health
For Infinite Health press start, then scroll to the right using the "R" button until you reach the Level Up screen. But you have to be able to level up manully! Then press level up. After that press the white button on your controller. Then a text messager should come up with a word bank. Simply press "A" to type in a letter, and use the "R" and "L" buttons to go side to side to choose your letter. Type in "YODA'S ROBE", then press the white button again to enter the cheat. Then you should hear a lightsaber. Cheat Complete!
Voice Change
With your primary controller plugged into the first port, plug a second controller into the fourth port. At any time, you can use the second controller to change the pitch of the voice acting.
Black Button: Lower pitch (Barry White)
White Button: High pitch (like Michael Jackson)
Watch Hidden Ending
Plug two controllers into your Xbox. Outside the doors to the final battle with Malak (don't open them!), push down both triggers and press the Y Button on both controllers at the same time. You'll need a third hand or a friend to pull this off.
Infinite Experience
In the dueling room on Korriban, there's a computer that lets you fight prisoners. Once you get your computer skill to the level that allows you to use 0 spikes to Hack Into Security Cameras or Hack Into Sith Database Files, just select that choice a few times and when you exit the computer interface, you'll get a load of experience.
Null Item Cheat
Before selecting a player, press X, X, Y, X, R, X, START. This doesn't work for point races.
Infinite Health, Spikes and Stims
Go to the Ebon Hawk and put all your medpacs in a Ebon Hawk storage container. After that talk to Jolee and have him make you more medpacs. Repeat this as many times as you want. And it is also a good way to earn credits.