Minecraft wiki entry for Defeating_a_dungeon, associated text: Zombie farms can offer tons of rotten flesh, but might also give the occasional carrot, potato or iron ingot. Random armor, iron swords or shovels might drop if the mob spawned with it.
If converted to a drowned farm, it gives rotten flesh and copper ingots.
Skeleton farms offer bones (for bone meal or bone blocks) and arrows, as well as the occasional armor or bows, which are sometimes enchanted.
Spider farms drop spider eyes and string.