Ghostwatch - watch or be watched
This is a Mod which allows Serveradmins to protect their server but still let people watch what players are building.
The Mod will detect every player on a whitelist in the config file and let them build normally in Survival.
If a Player not on the config whitelist joins the Server, he will be invisible, invulnerable and has the ability to fly, but he will not be able to break, place or interact with anything even the chat window won't open for them.
Mod abandoned as of 2017. Minecraft > 1.7.10 has the function build in.