Hidden Ending
(Note: You need two controllers for this trick)
With your primary controller plugged into ports 1, 2 or 3 have a second controller plugged into port 4. Before you enter the final battle with Darth Malak (do NOT open the door), pull both the 'L' and 'R' buttons on both controllers for a secret ending.

Voice Modulation
(Note: You need two controllers for this trick)
With your primary controller plugged into ports 1, 2 or 3 have a second controller plugged into port 4. While playign the game, press the black or white button on the second controller to raise and lower the pitch of the in-game voices.

Keep the Sith Armor
Once you obtain the armor and complete the associated quest, equip it to another party member when you select the option of handing it over.

Keep the Sandpeople Robes
Once you obtain the robes and complete the associated quest, equip it to another party member when you select the option of handing it over.

Easy Path to the Dark Side
When you discover the stowaway aboard the Ebon Hawk, select the option Get out of here, whoever you are! Right now!! Then, before she leaves, talk to here again and select the same option. Continue to repeat as you gain 1 Dark Side point each time.

Fun with Bastila's Wardrobe
Equip Bastila with dark robes and then remove them to see her in slightly different underwear.