You forgot the Foundation novels: Forward the Foundation, Second Foundation, and Foundation and Earth
Let me add one more both of you missed. Prelude to Foundation. And of course there are the Foundation novels and short stories written by other authors, noticable Foundation's Fear, Foundation and chaos, Foundation Triumph. |
Actually, all those books were mentioned earlier in this thread, especially since most people automatically include them in the original series.

The exception to this is the 2nd Foundation Trilogy (written by Bear, Benford, & Brin), which is rightly considered to be a separate series. I myself didn't care much for them, although Foundation & Chaos and Foundation's Triumph were okay. Foundation's Fear was particularly bad, however; it's easily one of the worst sci-fi novels I've ever read.
Still, the rest of the series is superb!