Wood
Wood is one of the three basic resources in Heroes (along with Gold and Ore). Wood is needed for most buildings, and thus, it is vital to claim a source of Wood as early in the game as possible. The primary source of Wood is the Sawmill, which brings its owner 2 units of Wood per day. You can also find Wood scattered around the land in small amounts (5-10 units of Wood per resource stack).
In Fortress, Castle, Necropolis and Stronghold towns you can also build the Resource Silo which will bring you 1 Wood and 1 Ore per day. Any hero equipped with the Inexhaustible Cart of Lumber gives you 1 Wood every day.