Darren Kuropatwa's insight:
"The Rational Number Project is an ongoing research project investigating student learning and teacher enhancement."
An excellent collection of "research into practice" resources for teachers to help the students learn about rational numbers and operations in mathematics.
While many resource are unavailable due to copyright (which the authors are working to release) there are also many freely available resouces for teachers to use with their classes and teachers & teacher trainers to use for their own professional development.