Paintbrush Glitch
In the game, paintbrushes have no weight or mass assigned to them, giving them many strange applications. For example, if you drop a paintbrush out of your inventory, it will float in mid air, and you can jump onto it and stand on it. Using multiple paintbrushes, you can make a kind of staircase, by dropping them one after another. This allows you to reach areas that are otherwise unattainable.
Boots of Springheel Jack
The Boots of Springheel Jack are very useful, but in the last mission of the Thieves' Guild, they are destroyed by a long fall. To prevent this from happening, go back down the tower where you first entered, and activate the fireplace in the sleeping quarters. Find an Imperial Guard on this same level of the tower, get caught by him, and elect to pay the fines. This will teleport you out of the tower, with both the boots and the completed quest. Alternatively, if your acrobatics skill is high enough, you can survive the fall at the end of the mission without wearing the boots, which will prevent their destruction.
Unlimited Arrows
This cheat allows you to garner an unlimited number of arrows of any kind, but requires that you be a Journeyman of Acrobatics. Equip a bow and draw an arrow back, but do not release it. Hold the "L" button, then press "Y" and move in any direction, in order to execute a combat roll. The bow will fire an arrow, but the arrow will also stay in your inventory. Retrieve the arrow that fired and repeat as many times as desired.
Item Cloning
You must have two different types of arrows and a bow to clone items. Equip the bow and begin to draw back an arrow. Open the inventory screen before you finish drawing the arrow back. Select the other set of arrows twice, and a message stating that you cannot equip these while attacking should come up. Then drop any item that you wish to duplicate. It will fall out of your inventory and will duplicate on the ground. Note that this only works if your game has not been patched with Xbox LIVE.