Arlo Guthrie's (Alan Moon's) 'Moon Song'
Very last scene of the series. They cleverly used dialogue that would work just in case the series was renewed...damned well should have been!
http://www.youtube.com/watch?v=CoZk0_g_Vd8
Since that video seems to be gone, here's another one that has a little more to it, including the graduation scene and the ending credits:
https://www.youtube.com/watch?v=u2TBNFRPUL4&index=45&list=PL90 9F61C9BF64ED92