WoWSniffer is basically a language hack, but because it doesnt touch the WoW process it is pretty much impossible to detect (assuming you run WoW with restricted privledges)
The source code is included!
The program has been added at the bottom of http://firepacket.net/WoW OR you can simply download below:
Direct Download: http://firepacket.net/WoW/WoWSniffer.rar
Source code: http://firepacket.net/WoW/WoWSniffer_src.rar