WebChest is an inventory editor for Minecraft. To start, simply choose a level.dat from a single player world, or a player data file from multiplayer and upload. If you're using Windows 7 (and maybe Vista), the location is at C:\Users\xxxxx\AppData\Roaming\.minecraft\saves\Worldx (AppData might be hidden), where xxxxx is your user name. One Mac, the location may be: ~/Library/Application support/minecraft/saves/Worldx. Substitute 'Worldx' with the world you wish to edit the inventory for.
Once you upload the file, you'll see your current inventory displayed. Simply drag items from the container on the right into your inventory where you want them. Click on an item in your inventory (except armor and tools) to edit the number of that item.
When you are done modifying your inventory, click the 'Download' link to download the edited file. Place this file back into the location of the original. BE SURE TO BACK UP YOUR ORIGINAL .. JUST IN CASE.
For more information, check out this the forum thread