Over the course of my PGCE and teacher training, and subsequently as a full-time Maths teacher, I've been building up a bunch of resources - interactive stuff as well as worksheets.  They're available here: Teaching Resources

However, during my degree and PGCE course I did a fair amount of research and wrote a bunch of papers on various education issues.  More applicable to those studying to become teachers rather than those who have already made the leap, they're available here:

Is Constructivism important for thinking about the learning of mathematics?  
Essay (pdf)

Should mathematics be compulsory?
Essay (pdf)
Lesson observation from the point of view of students.
Essay (pdf)
Relating a personal learning experience to a learning theory.
Essay (pdf)
How mathematical ideas are developed through school, higher education and the workplace.
Essay (pdf)
Lesson observation with particular reference to assessment methods
Essay (pdf)

Reflective essay on experience in schools
Essay (pdf)

Masters' Level Assignments
These are 5,000-word long essays which you may be interested in.  If you don't have the time, click the thumbnail beside them for a handy wordle.net summary!  
Theories of Learning (pdf)
(Wordle: Theories of Learning wordle)

Teaching and Learning Mathematics at A-level (pdf)
(Wordle: Teaching and Learning Mathematics at A-level)