Code::Blocks is one of the simplest yet powerful IDE for C/C++. I would recommend any beginners who like to learn C/C++ programming to use this. It is opensource, cross-platform, and best of all, free.
I recommend to download codeblocks-xx.xxmingw-setup.exe for Windows as it takes care of installing gnu c compiler for use with the IDE.
For other OS, it is straight forward options.