The core of the arcapos applications is written in C, so are most of the Lua modules (bindings to various hardware or protocols).
Game Pencil Engine
The Game Pencil Engine Editor is coded in C/C++.
Used in backend and clients for performance heavy tasks.
The Sqreen PHP agent is both a PHP extension, built in C, and a daemon built in Python.
Axis2/c, rampart/c, savan/c, barbershop and custom php extensions.