To copy an item; quick save the game, then drag an item out of your inventory, but not somewhere there is a red X. Without dropping first, quick load the game, then release the mouse button when it starts to load. This will produce a copy of the item on the ground, with the original still in your inventory.
There is a much easier method for items which can be stacked (such as gold);
Drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.
For your primary stats, experience and skills (ie data.000) you can use
iZakaroN's SaveEditor.
Secondary stats can not be edited directly, but depend on the primary stats.
For item editing (items.000) check out
DAD's item editor.
If you really want to poke around with a hex editor, this topic contains more information on item editing;
please help with my blind heroI have seen
ArtMoney (a memory editor) recommended for cheating, but have not tried it myself.
