Item Duping (One Computer)
Enter multiplayer and fill your belt with potions (mana or health, it doesn't matter). Drop the item you wish to dupe and walk a fair distance away from the item. Click on the item you just dropped and your character will start walking toward it to pick it up. Just as he picks it up, click on one of the potions in your belt. Drop the potion after you picked up the item, and the potion will turn into the item you already picked up.
Item Duping (Two Computers)
Obtain the item you wish to dupe, then save your file. Copy the Save folder from "Diablo II" and paste it to your desktop. Long on to Open BattleNet with both computers and meet each other in the same multiplayer instance. Give the items to the second computer character, save then exit. Hold the "Alt" button and press "Tab" to go to your desktop; copy the Save folder from there and paste it back to its original location. Meet the second computer character back in the multiplayer game, and you will both have the items exchanged. Give the items back to the first character.
Unlimited Imbued Items
Download the version 1.00 patch only. Play through the story mode long enough for Charsi to offer the imbuing of items for you. Save and exit; downloand the version 1.03 patch, then install it into the game. Join a BattleNet game, imbue something, then give it to a friend (or another computer's character if you have one). After you exit the game and then return to the same character, you will be able to imbue again.