There are several freely available BDI systems on the WWW; here I provide links to some of the best know. Note that systems are listed in alphabetical order. If you know of other BDI systems, please send me the details (including full working URL).
JAM started off as an implementation of the PRS BDI agent model in Java, but has grown considerably. Implemented as an interpreted programming language, Jam enables you to build agents that communicate and migrate. It integrates fairly easily with legacy Java code via the Java reflection package. JAM was implemented by Marcus Huber's consulting company Intelligent Reasoning Systems, but is available on a GNU-style licence.
The original PRS system was implemented at Stanford Research Institute's AI center in LISP, and this system, implemented in common LISP, is a direct descendent. PRS-CL has been used in space shuttle fault diagnosis, aircraft maintenance, and mobile robotics, and has associated with it several graphical tools (e.g., a plan editor) to make your life as agent programmer easier. Like JAM! and UM-PRS, it acts as an interpreted programming language. As far as I am aware, SRI do not make PRS-CL freely available; I suggest you contact Karen Myers if you wish to find out more.
UM-PRS is a pre-cursor of the JAM! system described elsewhere on this page. Developed by Marcus Huber and colleagues at the University of Michigan, it is essentially a re-implementation of the PRS system in C++. Like JAM!, it is an interpeted language for programming agents, which allows you to hook into legacy C/C++ code. It is freely available via a GNU-style licence, but I believe it is no longer supported.