When considering which hard armor plate to purchase, the only option other than ceramic and steel is currently polyethylene. It is a lightweight unidirectional material that resists bullets itself without spall of kevlar backing. It is 50% lighter than ceramic plates. They have been around since the late 1980s, the technology to make polyethylene plates thin enough for modern use is recent. High-pressure treatment increases performance and decreases weight.
The word polyethylene may not be as common as ceramic, but it's not as strange a substance as it sounds. It is a modified type of plastic, the same as your milk jug. But no armor plates are made of milk jugs. There are many different forms of polyethylene, including woven and shield materials.
Unlike ceramic, these plates take advantage of the rotation of a round to slow it down. As the bullet's friction creates heat, which partially melts the polyethylene until it stops the bullet. Once the bullet slows and eventually stops, the polyethylene cools and re-hardens. Similar to a self-cauterizing wound.
Because of the way in which polyethylene plates work, they can stop multiple bullets. You can pretty much put in as many bullets as you can fit on the plate because it doesn't impact a very large surrounding area when the bullet hits. And because the material is more resilient than a hard material like ceramic, dropping a polyethylene plate will not cause damage. They are also relatively light, weighing 3.5 pounds for a NIJ rated LVL III 10x12-inch plate.
Polyethylene plates have their own downsides, however. For example, technology doesn't exist to create a commercial type 4 plate made entirely of polyethylene yet. It would require some ceramic to be added to the plate. Price is also a factor. Polyethylene plates cost approximately 25% - 50% more than comparable ceramic plates.