An addon that lets players view possible enchants a player might want based on items being traded in the screen. Also, an enchanter can click the enchant onto the person's gear assuming they have it learned.
The gold standard of all things Waylaid and Writs. This addon puts the ease of not only gathering your items for the cheapest cost, but also plans your route from one turn in to the next.
Chat notifications for trading players (proof of trade).
Raid buff monitoring and announcements