What is the limit in terms of power, for PoE?

I'm fairly new to the concept of PoE, but I like the idea a lot since I want to try and install as many devices in my new house build that can work over this technology as part of a home automation system.

What is the maximum power (voltage, watts) that PoE can deliver?  I know its not high powered, so I am guessing you can never get an electric shock from it if you accidentally cut the cable?

Apart from security cameras, what other devices can now be powered by this route?

When did PoE start to become more prominent and what are the advantages / disadvantages of it?

Thx.