Zombies will rarely drop iron ingots, carrots, potatoes, and any naturally spawned equipment. Any picked-up equipment has 100% chance of dropping and will drop with the same damage level it had when picked up, while any naturally spawned equipment has a 8.5% (9.5% with Looting I, 10.5% with Looting II and 11.5% with Looting III) chance of dropping and will drop with a random durability.
A zombie will drop its head if killed by a charged creeper's explosion.
Zombies drop 5 experience when killed by a player or tamed wolf, and an extra 1-3 extra experience if the zombie has equipment. If the zombie is a baby, it will drop and additional 7 experience, except in the console edition where baby zombies drop no experience.
In the Pocket Edition, zombies will not drop iron ingots at all.