INVedit is probably also one of the most essential tools for anyone who is dedicated to Minecraft. This is a seperate program which loads up the inventory from one of your worlds and shows you what items you have as well as a list of what items you can add to your inventory. INVedit can also manipulate the durability of your items, allowing you to restore your lovely diamond armor back to its original state! I do not reccomend this program for someone who just started playing Minecraft because I honestly believe that it can take the fun out of the survivability aspect of the game, especially when you're able to give yourself infinite obsidian, diamond weapons and TNT.
Notes: When you open the program, click on the Open button at the top, pick your world and once you've added/modified the items in your inventory, go to the Save button and save it to the world that you want. Remember to save your game right before you edit your inventory otherwise it won't actually work.