I've studied C++ as a college course. And I have been working on it for last three years. So I kinda have a fairly good idea what is it about. But I believe to know a language is quite different from using it to full potential. My current job doesn't allow me to explore much.
I have seen you guys suggest studying a open source project and perhaps contributing to one too. So my question is, can you suggest one(of both?) to start with that is simple and doesn't overwhelm a starter.
I was in the same situation 12 years ago when I got my first programming job out of college. I didn't do any open source but I managed to gain a lot of practical and advanced C++ knowledge by reading books (the dead tree kind).
In particular, there was an excellent series by Scott Meyers which I feel helped the most in turning me from newbie to professional:
Effective C++: 55 Specific Ways to Improve Your Programs and Designs
More Effective C++: 35 New Ways to Improve Your Programs and Designs
Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library
The topics in these books range from beginner to advanced. It took me about 2 years working in C++ to understand every chapter in these books, so don't be disheartened if it goes over your head at some point... just try reading it again later :)
If you're willing to climb the very steep learning curve, I would think that you could do worse than studying parts of the boost libraries. I don't think there is another similar body of C++ code out there that is pushing the boundaries of what can be done in C++.
Obviously learning something like this does not happen overnight; You can study existing open-source projects like other posters suggested. However, I have one more suggestion: I suggest joining one of the big IRC c++ channels (for example #c++ on efnet or freenode). Of course most of the questions / discussions going there are basic; however quite often interesting discussions and questions come up. Over the several years I've learned quite a bit just by participating in channel discussions.
My main advice would be that you do not need to stick to C++ to learn technique.. Study Erlang, Lua, and Haskell (lots of others). You will learn so much from these language that you can apply in C++.
Google protocol buffers is a very well written C++ application that sticks to the features of C++ that are proven in the industry -- i.e., they don't go over the top with features that cause headaches for using the code.
In terms of learning to use C++ to its full potential, well the full potential is often hidden. There are very few exceptional C++ programmers, even in the open-source community. So I suggest you read some books. Like the books from Scot Myers and Herb Sutter (all 6 of em).
Boost is an amazing library and has a lot of good code, ranging from simple techniques to very advanced techniques. It definitely touches all of the bleeding edge of C++ idioms. The problem with boost is, it doesn't work well with modern IDE features (code-completion being the main problem), and it has a LOT of symbols :D. It is the place to learn about generic programming and template meta-programming, the former represents the most advanced and immediately-usable idioms, and the latter you can skip for the immediate future.
Apart from the Scott Meyers books DSO has already suggested: Participate on SO and try to answer other people's C++ questions. If you don't know the answer, wait a little until some good answers are posted. You will probably get some idea where the gaps in your knowledge are and you could do some more reading to fill those afterwards.
