Bobick - rotating Objects Using Quaternions

Bobick's article, published on Gamasutra, explains the benefits of
representing orientation of an object with quaternions.  The paper doesn't
necessarily solve a particular problem so much as it gives an informative
overview of quaternion theory and what the benefits to using it over other
methods are.  Bobick explains the limitations of matrices and Euler angles,
such as gimbal lock, and how quaternions can be used to overcome these
problems.  In addition, he also covers methods of converting rotation
matrices and Euler angles to and from quaternions.  The paper is very
straight-forward and even provides code snippets for doing the data
conversions.  Overall, it is a very easy paper to understand.