Where can I get an open source implementation of the Parks-McClellan optimal FIR filter design algorithm?
There is an open-source implementation of Parks-McClellan (also known as the Remez exchange algorithm) in GNU Octave, a free-software implementation of a MATLAB-like environment. The function, called "remez", is contained in the "signal" package, which is hosted at Octave-Forge. If you download the package, you'll find "remez.cc", a C++ implementation of the algorithm.

One nice thing about Octave is that it is almost code-compatible with MATLAB, so you can easily port code over to use there if you like. It's a good way to peek under the hood at implementations of algorithms that are provided in MEX form in MATLAB.