documentation for STL
Here is the reference I'm using. SGI , Offline Download
Here is another reference
In general, it is best to use the documentation that comes with your C++ toolchain. For general-purpose docs, I like the GNU libstdc++ documentation.
If you're looking for a proper reference, then, truly, nothing can beat "ISO/IEC 14882:2003 - Programming Language C++" - after all, it's the primary source. I'm not aware of any legal way to get the PDF for that for free. You can buy the PDF from ISO, but they ask ~$300 for that, way too much in my opinion. A cheaper option is to go to one of the national standard bodies that make ISO - they republish those standards under their own name (but otherwise unchanged), and usually the prices are more sane. The cheapest paper version I'm aware of is published by British Standards Institute - available on Amazon for $85. The cheapest download PDF seems to be $40 from the shop of the Australian member organization.
cplusplus is an easy to read up reference.
STL at Codeproject. Make sure you check the date of the article.
C++ Standard Library at MSDN.
If you want dead trees, maybe you'd be better off with a proper book? I found this one indispensable: The C++ Standard Library: A Tutorial and Reference by Nicolai M. Josuttis