Would anyone recommend certain open-source projects written in C/C++ to study? What I mean is reasonably-sized projects or something like that, that would be a good project or good projects to read through? It might seem like a lazy question in a way, but I'm wondering if you spend some time studying other people's code, what do you normally study, or what do you plan on studying, or what would you study?
The most obvious answer for C projects would be GNU, probably... but I'm thinking of easier, simpler code, but not something as simple or Microsoft-centered, say, as planet-source-code.com or similar sites.

Did you think of selecting something from http://sourceforge.net/ ?

Well, like I said, I'm thinking of projects or programs that are simpler, easier, smaller, etc. I've already thought of sourceforge, I'm just not sure they have any reasonably-sized projects or programs really. I think they focus more on large, complex projects.

there is this " CImg.h " i use it a lot
http://cimg.sourceforge.net/
see the screenshots page( part of the tutorial )
http://cimg.sourceforge.net/screenshots.shtml

Please OP does not consider this a good example of C++ source code.

I can not give any concrete examples as I do not know your level of C++ knowledge but personally I do read lots of other peoples code. It helps to understand a different style and thought process, shows clean and unclean methods of achieving objectives, makes you think about readability of the source and what could be done to improve it.