A local rednet message sniffer; prints the host computer name and message content.

CC Rednet Sniffer.lua

-- ComputerCraft Broadcast Rednet Monitor
--   Monitor all traffic on local broadcast RedNet
-- by Alexander "SquidLord" Williams (SaladinVrai)
 
 -- The MIT License (MIT)

-- Copyright (c) 2012 Alexander Williams

-- Permission is hereby granted, free of charge, to any person obtaining a copy
-- of this software and associated documentation files (the "Software"), to deal
-- in the Software without restriction, including without limitation the rights
-- to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-- copies of the Software, and to permit persons to whom the Software is
-- furnished to do so, subject to the following conditions:

-- The above copyright notice and this permission notice shall be included in all
-- copies or substantial portions of the Software.

-- THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-- IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-- FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-- AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-- LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-- SOFTWARE.
 
-- Get the system label for identification
 
local SysName = os.getComputerLabel()
 
-- Iterate over all sides for rednet.open
 
for n,m in ipairs(rs.getSides()) do
  rednet.open(m)
end
 
-- Identify
 
print("SquidOS RedNet Sniffer ...")
if SysName then
  print("  Running on node: " .. os.getComputerLabel())
else
  write("  Running on ")
  textutils.slowPrint("UNIDENTIFIED NODE")
end
 
print()
print("Press and hold Ctrl-T to end.")
print()
 
-- Loop forever (or until forced termination with Ctrl-T)
 
while true do
  Event, Id, Content = os.pullEvent()
  if Event == "rednet_message" then
    print(Id .. " (" .. textutils.formatTime(os.time(), false) ..")>")
    print("  " .. Content)
  end
end

Hi!
I modified your gist a bit to respond to button press to stop sniffing and also to close networks.
If you like it, update yours, please :)
https://gist.github.com/complover116/5404622

You should use modem api to get the messages sent from all messages not just rednet broadcasts