GAptI is currently a simple utility to handle a .apt file as defined by ThirdPartyApt.
The idea is simple. It pops up a dialog asking if the user is sure he wants to install the software, then prompts for crediantials to fulfill it. A source is added to apt/sources.list to track future updates.
Currently it lives at http://akita.larvalstage.net/~wasabi/bzr/gapti--main.
Included in universe since 2006, april 26.