gems, worked jewelry (gnomes, svirfneblin) 
     stone for construction (dwarves, duergar) 
     metal ore, worked metal (dwarves) 
     minerals (dwarves, duergar, svirfneblin, gnomes, drow) 
     mushrooms -- exotic varieties (duergar, drow) 
     poisons (drow) 
     slaves (drow, illithids, other unknown sources) 
     weapons (dwarves, duergar, drow) 
     armor (dwarves, duergar, drow) 
     silk (drow) 

 Likewise, there are items that can only be imported into the Underdark: 

     wood and wooden products, such as paper 
     spices for food 
     wine and ale 
     cloth and furs 
     produce (surface fruits and veggies) 
     news of the surface world 
     perfume 

These products are typically brought to the Underdark by trading companies run by surface-dwellers.  Many trading companies have hidden and well-defended warehouses near connections to the surface.  Acquiring and trading goods here can be a profitable business for someone with the courage and skill to survive in the savage depths of the Underdark.