IP Broacaster displays your local (LAN) and internet (WAN) IP numbers in the menubar. Includes options to toggle wan and lan display.
Also see IP Scanner for Macintosh
Download IP Broadcaster 1.4.1 (free)
Intuitive, award-wining media and network software featuring IP Scanner, DropCopy, Video Playlist Manager and more for Macintosh, iPhone, iPad and iPod Touch