Item Viewers
Item Viewers gather all Items in the game and displays them in a grid next to your Inventory, and provides means to search for Items.
Left-Clicking on any Item will show how to obtain it. For example, clicking a Diamond Sword will show it's crafting recipe.
Right-Clicking on any Item will show Recipes that include the Item.
Pressing A on any Item will add it to the Favorites list, which appears on the left side of the Inventory.
Pressing A again will remove it from the Favorites list.
Search Features
There are several extra ways to search Items. For example, Prefixes:
@ Searches by Mod Name
$ Searches by Tag name, or OreDict for 1.12.2 and older.
% Searches by Creative Tab.
# Searches for text in Tooltips.
Additionally, typing | between two sets of text will search for both at the same time.
Example usages:
@mekanism Shows all Items from the Mekanism mod.
@minecraft ingot Shows all Items from Minecraft that include the word "Ingot".
$minecraft:block/banners Shows all Banners from the Banners tag.
Just Enough Items (JEI)
Just Enough Items is the most popular option (and the most downloaded Mod).
It is incredibly basic, providing the above features, and a Cheat mode.
EMI
EMI is a modern and elegant Item Viewer that optionally JEI as a backend.
Its UI is extremely customizable, adding the option to have 4 different panels to the Inventory at once to display all Items, Favorites, Craftable Items and Viewed Items / Crafted Items.
It also prominently features a Recipe Tree, which shows all Items needed to craft something.
Recipe outputs can also be favorite to craft or view quickly.
Roughly Enough Items (REI)
Roughly Enough Items is another modern and elegant Item Viewer with some extra features over JEI.
Includes Regex search. For example r/(Golden )?Apple) to search for Golden or regular Apples.
Recipe Viewer has increased depth, and Recipes can be highlighted to the left when pressing A on blank space in the Recipe.
A Keybind can be set that can be held while the Recipe Viewer is open to export a Recipe as an Image.
Had Enough Items (HEI)
Had Enough Items is a spin-off from JEI for 1.12.2 focusing on performance, stability and ease of use.