If you are anything like me you like leveraging implementations that are publicly available and not re-intenting the wheel all the time. Here are three source code search engines to help you with that:
Personally, the tool I use is plain vainilla Google but I have used Koders from time to time. Now Google Code Search comes along (it was released just today) so I'll be sure to check that out. I didn't even hear about Krugle before today but it looks slick - I will check it out as well.
While I'm on the subject of use of open source I can't but mention the absolutely best open source C++ library out there: Boost!