KDevelop

KDevelop is a cross-platform IDE that I primarily use for all my C++ IDE needs.

Some notable contributions:
Scratchpad plugin. A plugin that helps you keep a list of “scratches”, temporary code files that don’t belong to a project that you can run. Helps you experiment with code, or run small programs that don’t need a full project/build system.
Assistant to automatically generate header guard if they’re missing in header files