Multicast IP addresses

I've known about multicast IP addresses for a long time, but just
became interested in them. Questions:

 % Is multicast theoretical, or actually being used?

 % What multicast IP addresses broadcast free/test information (time
 of day, weather, delayed stock quotes, etc)? I think I've figured out
 how to register myself as a listener to a given multicast IP address,
 but which IP addresses are actually "broadcasting" anything?

 % What command-line app lets me register myself as a listener and
 listen to all traffic on a given multicast IP address?

 % Sending data to a multicast IP address is easy: just send a UDP
 packet to the address. Question: how do multicast IP address users
 prevent people from spamming a given IP address?

 % Anyone can listen to a multicast IP address w/o the sender knowing
 it. How do senders transmit non-public data over multicast IP?
 Encryption?

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