A post to the debian-user mailing list included a nice little aptitude command to show installed packages not from the main debian apt repositories.

mshuler@ares:~$ aptitude search '~i!~Odebian'
i dynagen - Cisco 7200 Router Emulator Command Line Interface
i dynamips - Cisco 7200/3600/3725/3745/2691 Router Emulator
i w32codecs - win32 binary codecs