SessionInstaller allows applications to easily install additional software (e.g. extensions or GSreamer codecs) by calling a simple distribution neutral D-Bus interface.
The confirmation, error reporting and progress notification of the installation is handled by sessioninstaller. Currently it comes only with a GTK based user interface.
The D-Bus interface is developed under the PackageKit umbrella and is available on the session bus. The reference implementation can be found in gnome-packagekit. It was also adpoted by KPackageKit.
In contrast to gnome-packagekit and KPackageKit it doesn't use the PackageKit daemon for querying and installation, but instead makes use of APT and aptdaemon directly (alternatively synaptic can also be used).
There is a screencast video for demonstration.