This is an excellent client mod. TooManyItems is an in-game inventory editor. If you're not sure what that is, the screenshot may help. It gives you a graphic overlay on your inventory screen, containing a list of every material in the game. Left and right item clicks will place stacks of 64 in your inventory, or add items one at a time. This has the same effect as the /give and /i commands of multiplayer servers. Our server is compatible with TooManyItems.
When playing single player, TMI gives you access to all of the multiplayer features, and many more, including: saving and loading inventories, item deletion, infinite stacks, and fast transfer.
Once you have the mod installed, pressing the "o" key while viewing your inventory screen will toggle the TMI overlay on and off.