Cameras, network switches and routers (smaller ones), access points, access control units (PoE power can be used for powering the control unit and the door lock actuation), alarm system components (keypads, motion sensors, etc.), door intercoms, general purpose I/O relay units (for controlling everything from blinds to motorized screens, etc.), edge compute devices, powered speakers for music / intercom use (power is entirely supplied via PoE), dongles for pushing USB / video, etc. over Cat5e, etc.
For security, I'd highly recommend Axis cameras (I've done a number of installs with Axis cameras and IP-based access control) and while a little more expensive, it's well worth it.