One of the most popular mod that everyone should have it installed is Too Many Items Mod. Too Many Items Mod is developed by Marglyph, a cool guy who made Minecraft a better game. I want to make this really simple. So I probably say that Too Many Items Mod is a mod where player can create, delete, manage their inventory in-game. You spawn any items, blocks, or anything from other mods.
How to Use Too Many Items Mod for Minecraft 1.0.0
Press “O” on keyboard to toggle on and off
Shift-left-click on the item on sidebar to add unlimited stack to it
You can save states to bring it back later
If you want to use in multiplayer, you have to be op.
How to Install Too Many Items Mod for Minecraft 1.0.0
Open Start Menu -> Run
Type in %appdata%
Right-click on minecraft.jar and choose open with WinRAR or 7Zip