This question is a lot like Good book for learning the C++ standard template library? but aimed at the cash-strapped student type. I remember searching for some reference material about the STL a while back, but I couldn't find any good links.
Are there any worthwile free tutorials / references / best-practises available?
CPlusPlus.com has a pretty readable, though incomplete, STL reference.
The Dinkumware Compleat Reference is complete and detailed, but possibly tough for a beginner.
See also the Best C++ Resource question.
Volume 2 of Bruce Eckel's Thinking in C++ is a good place to start. It's also free!
For quick references, I often go turn myself to cppreference.com.
I always find what I need there...
I know that you mentioned "free" but I believe that learning to use the STL, particularly mastering the concepts behind it, is useful enough to warrant the purchase of, and the time spent reading, a book. To that end, I would highly recommend "The C++ Standard Library" by Nicolai Josuttis.
The C++ Standard Library: A Tutorial and Reference is an excellent book for both learning and reference. Perhaps you can barrow it from a friend or library.
STL Tutorial and Reference Guide: C++ Programming with the Standard Template Library is a good book, but personally when developing and I need to know anything about the STL I keep the libstdc++ API documentation open next to me, but this won't teach you anything about generic programming though.
This book helped me a lot understanding the "STL way":
Generic Programming and the STL: Using and Extending the C++ Standard Template Library
And it is also a really great reference once you learned how to use the STL.