Recommendations for Fast Multipole Method implementation?

I am by no means an expert in FMM, but this java implementation and introduction is the best source I've found so far for explaining it carefully and slowly. The paper is good at defining terms before using them, and the code at least is useful as a reference point. The math still gets hairy very quickly, but it is what it is :)

A pedestrian introduction to fast multipole methods is a close second. It doesn't explain the actual details of a working FMM implementation, but it's a good introduction to the basic ideas.

I like the short course on FMM. In begins with FMM in 1D, than it uses theory of complex variable to do FMM in 2D. And than there is the crazy 3D version which uses theory of spherical harmonics functions, which I guess can be very difficult for non-mathematician. But If you need FMM only in 2D you should be fine.

Unfortunately no pseudo codes are given there.

But do you really need the accuracy of FMM?. You might be fine with Barnes-Hut's algorithm