This isn't really a cheat, but when you turn on the debug menu, select Mos Espa. You will be able to kill anyone in the city and not be able to get killed. It's pretty fun.
Glitch: fall through level
In case you just love the sound of Obi-Wan dying,in the gardens of theed, you can fall through the level. Here's how you do it. Go to the garden area with all of the droids (recommended you have invincible on)and walk around the walls. Stop when you find a place in one of the corners that sort of looks like a very thin bowling ally (2) and jump onto it. You will fall through and die.