TooManyItems is an inventory mod that allows you to create items in both single and multi-player (multi-player mode uses the /give command and requires you to be a server op, of course).

Basic operation: When you open your inventory screen, left-click on the items on the right to add a full stack, or right-click to add 1 at a time. Press the "o" key to turn TMI on and off.

To install:

Backup ..appdata\.minecraft
Find minecraft.jar (it’s in ..appdata\.minecraft\bin)
Open it with winzip or similar
Add the contents of, minus the file
Delete the folder “meta-inf” from the archive
Save the archive
Run Minecraft and hit “o” while in the inventory to turn TooManyItems on.