Writing a "recommendation letter" for a supervisor from a student? As a thank you to my thesis supervisor, I would like to write some sort of letter expressing my gratitude for the work he has done for me.

I was wondering if students ever write letters in such a way that they can be used by professors as a sort of "reference" letter when applying for jobs or teaching grants. For reference, my supervisor is a Senior Lecturer at a university in the Australian system. Would a letter like this be appreciated? What should it look like?
I don't know about the Australian system; in the US, it's quite normal for applications for promotion and teaching awards to require letters from students (though I think usually students from classes, rather than doctoral students).  However, I wouldn't pre-emptively write one; my recommendation is that you write a letter or email, in which you could offer if they ever need such a letter.  The other thing you could reasonably do is write a short email to their head of department, just saying in a few sentences wy you felt that they were successful as a supervisor.  That's probably the best way to help their career.