D&D General Best D&D product of any edition (non-core book)

[Lakesidefantasy]

Unearthed Arcana both 1st and 3rd editions.

Manual of the Planes from 3rd edition.

 

[Dragonhelm]

I always liked this booklet and got a few campaigns out of it. I found it to be a good style and formed what I think of Forgotten Realms (FR). First one I thought of reading the thread title.

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This was one of the first D&D products I owned. It spawned my first campaign, set in Daggerford. I absolutely love the description of the town and NPCs.

The only downside is that there are issues with the map for Illefarn. Other than that, it's rock solid.