rigid body dynamics?
If we understand you right, you are looking for a library of building blocks for constructing a rigid-body system's equations of motion. Yes, we agree, normally, the most challenging portion of solving such a dynamics problem is to establish the equations of motion. Once such equations are created, one will very quickly give up on solving them by hand but numerical methods for solving such a system is widely available.
One of the most general methods is the energy method (Lagrange Method). The outline of the method is all over the textbooks but the application of the method to any real problem is very cumbersome. An easier way, although still not trivial, was developed by Professor Thomas Kane of Stanford University. Based on Kane's method, Kane and his former students developed a software called Autolev that automates the process of building the equations of motion. All you really have to do is create an input file that describes the system. If anyone is interested, here is the link.
We will evaluate the feasibility of building a small library that might help users create equations of motion for simple systems.