I often mention books that I use as resources, but rarely websites. It seems, on average, that online C++ resources are lacking. However, there are a few that I have found that provide quite valuable insight.

1. cplusplus.com language reference. I am particularly fond of the iostreams reference.
2. SGI’s STL reference. It’s an oldie, but a goody.
3. Stroustrup’s C++ Style and Technique FAQ. It’s a fascinating look into the language through the mind of the creator.
4. C++ FAQ Lite. Extensive collection of C++ questions and answers. Some of the answers are lite on detail, but you cannot ignore the wealth of information available.