There is a multiplayer item dupe that I tried back in BETA and it worked like a charm, you would have to..
Step 1 place a cafting table,
Step 2 dig a 2 block deep hole and fill it with water.
Step 3 Put your desired item in the cafting table afte jumping into the water (so that you start drowning while in the cafting table)
[Note]; for a stackable item you need a full stack, ex; you will need to put a full stack of iron in the crafting table
keep it open until you die.
Step 4 When you die return to your deathpoint and pick up your item(s) you should have a full stack +1 that extra one will be a bugged block meaning
that you can place it however many times you want and it woont run out.
or you can right click free spots in your inventory to make more blocks.
NOTE; this also worked with non stackables.
just you only had to put the single item in.
Im not sure if this is patched by bukkit not though

sorry... I dont wish to ty it online as i do not dupe anymore ahaha