Plugin channels

Plugin channels are implemented using packet 0xFA, allowing client mods and server plugins to communicate without cluttering up chat. This post by Dinnerbone is a good introduction and basic documentation.

Channels internal to Minecraft

As of 1.3, Minecraft itself started using plugin channels to implement new features. These internal channel names are prefixed by MC|.

MC|BEdit

Client to server

When a player edits an unsigned book.

MC|BSign

Client to server

When a player signs a book.

MC|TPack

Two-Way

Remote texture packs.

MC|TrList

Two-Way

The list of trades a villager NPC is offering.

MC|TrSel

Client to server

When a player selects a specific trade offered by a villager NPC.

Notable community plugin channels

Channels listed in this section are not Mojang-sanctioned. This is just a likely-incomplete list of channels used by mods/plugins popular within the Minecraft community.