The first time we heard about Motorola's plans to launch a phone with modular back plates, they were called Amps and there were going to be at least six of them. However, Motorola only announced four "Moto Mods" with the new Moto Z today. You'll be able to get stereo speakers, a projector, a battery, or a pretty back cover.
Let's hit these one at a time, starting with the JBL SoundBoost. It weighs in at 145g and is 13mm thick, so it'll make the phone quite a bit larger. It has 2 27mm speakers at 3W each. The internal 1000mAh battery gives it about 10 hours of life and it charges over USB type-C. This is the only one with a price on Verizon's site—$79.99.
The Moto Insta-Share Projector is a pico-projector attached to your phone, which seems sub-optimal to me. Wouldn't casting to a screen or projector be better than giving up your phone? It's 11mm thick and weighs 120g. The DLP projector has a resolution of 854x480 and can produce an image up to 70-inches with 50 lumens of brightness. The built-in 1100mAh battery is good for an hour of projection time and it charges over Type-C.
Motorola's Incipio offGRID Power Pack seems like the most appealing Mod. It adds a 2200mAh battery and 6.2mm of thickness. There's a standard and wireless charging version of this battery that weigh 79g and 85g, respectively. The wireless version works with both Qi and PMA pads.
Finally, there's the Style Shell. It's just a simple cover that lets you change up the look of your phone, rather like the 2015 Moto G.
For the more complicated Mods, Motorola/Lenovo are launching a developer program to help third-parties make new Moto Mods. Maybe something cool will come of it.