libneural neural net library: home page | tar GZ
UNH CMAC code: home page | tar GZ -- Note: you might want to test this before relying on it. I had some problems.
GPC++ genetic programming package: home page | tar GZ | docs
KNN: home page | C
C4.5: home page | tar GZ
AutoClass (Bayesian classifier): home page | tar GZ
CViz (a visualization tool designed for analyzing high-dimensional data): home page
MIT Lincoln Laboratory LNKnet Pattern Classification Software

ML problems

Mountain car learner
Pole balancer
Double arm pendulum simulator (just the system dynamics, in C)
Double arm pendulum simulator (wxWindows based GUI)
Partitioned efficient value iterator

Miscellaneous

UC Irvine ML database
MPIch 1.2.3 (Unix flavor)
Code and data for the Machine Learning textbook