This is a browseable, searchable in-game item database, with the caveat that it starts out empty.
It automatically adds item information that it picks from a number of different places: your character's inventory, other characters' inventories whenever you target them, links in chat, items in your bank, etc. LootLink will also color the links in loot messages with the correct rarity color, if the item is in its database. To browse or search, you'll need to use /lootlink (or /ll) to toggle the LootLink window or bind a key to the Toggle LootLink binding. I use NumPad /, but you can use whatever you want. Once you toggle the window open, you can sort the items by a number of different criteria, can scroll through the list and see tooltips for each of the items, and can insert a link to an item into chat by shift-left clicking it in the list. You can use the Refresh button to refresh the window (new items aren't automatically added to the window, though they're in the database, to prevent additions from becoming too confusing when the list is sorted). Finally, the Search button will bring up a dialog box that lets you specify all sorts of criteria or reset the current search parameters to the default. The search options are quite powerful, letting you find things like all rare rings that modify intellect and have level requirements between 10 and 40 or all shields with armor more than 200, block more than 20 and a level requirement of no more than 50.
Use /ll help or /lootlink help to get a list of commands.