Oxe Software has updated the free and Open Source Oxe FM Synth to v1.3.1, which sees the release of a native Linux VST version along with a stability bug-fix for Windows.
The Linux version was made without frameworks, using the Xlib directly. The code of both versions follow the same structure and is a good starting point for anyone who wants to learn how to do the same. The binaries (Windows and Linux, 32 and 64-bit) are available now. The source code is on GitHub.