Libbonoboui

A way to embed widgets managed by one application into another application.

This is not too difficult. For example, to embed a PDF viewing component (in C):

GtkWidget *widget;
widget = bonobo_widget_new_control ("OAFIID:GNOME_PDF_Server");
gtk_container_add (GTK_CONTAINER (container), frame_widget);
gtk_widget_show (frame_widget);

For good example code of how to effectively use bonoboui and pluggable components, have a look at GEdit.