Not Enough Items (NEI) is a forge-compatible mod that list all items currently in the game, including mod items. It is shown to the left and right of your inventory when open. It can be hidden by pressing the "O" key. By going to options in the bottom-left, you can cycle between three different modes: Cheat Mode, Utlity Mode, and Recipe Mode
Cheat mode allows you to spawn in any item by clicking on it's icon on the list on the right. Right-clicking will spawn a stack, and shift-clicking will spawn an infinite stack (shown as 111) best used for getting Butter. Additionaly, several buttons appear on the top left of the NEI window in cheat mode. Clicking the sun, moon, and cloud pictures will change the time to day, change the time to night, and turn rain on or off, respectively. The "C" button will turn create mode on or off, and the magnet will turn magnet mode on or off. Magnet mode pulls all entities within several chunks into your inventory.
'X' opens either a 'Locked' or 'Unlocked' enchanting grid. (Unlocked giving you the option of having every enchantment possible and incompatible enchantments)
'R' - Displays the recipe for the item.
'U' - Displays the recipies the item is used in.
If 'Creative+' is activated, it acts like creative mode but with a much bigger survival inventory.
In recipe mode, the player still has the same GUI, however, instead of giving you the items it will instead show you the recipes of the item.