Using GTK on X windows (i.e. Linux, FreeBSD, Solaris), you can use the XEMBED protocol to embed widgets using gtk.Socket. Unfortunately, the application that you're launching has to explicitly support it so that you can tell it to embed itself. Some applications don't support this. Notably, I can't find a way to do it with Firefox.
Nonetheless, here's a sample program that will run either an X terminal or an Emacs session inside a GTK window:
import os
import gtk
from gtk import Socket, Button, Window, VBox, HBox
w = Window()
e = Button("Emacs")
x = Button("XTerm")
s = Socket()
v = VBox()
h = HBox()
w.add(v)
v.add(s)
h.add(e)
h.add(x)
v.pack_start(h, expand=False)
def runemacs(btn):
x.set_sensitive(False); e.set_sensitive(False)
os.spawnlp(os.P_NOWAIT, "emacs",
"emacs", "--parent-id", str(s.get_id()))
def runxterm(btn):
x.set_sensitive(False); e.set_sensitive(False)
os.spawnlp(os.P_NOWAIT, "xterm",
"xterm", "-into", str(s.get_id()))
e.connect('clicked', runemacs)
x.connect('clicked', runxterm)
w.show_all()
gtk.main()