The VST plugins listed on this site's download page are all open source programs.
The plugins are built on top of Steinbergs VST SDK and can thus be built for Windows, MacOS and Linux platforms. They run in any VST3.0 compliant host/DAW (which include Bitwig Studio, Reaper, Plogue Bidule, FL Studio, etc.). Additionally, they can be built as VST2.4 plugins (that's for all you Ableton Live users!).
Both instruments and sound processors are available, both having a similar interface (the root vst.cpp acting as the entry point of the plugin, maintaining its model state and delegating the view logic to controller.cpp and the actual process to a dedicated processor class).
The plugins can be compiled for both 32-bit and 64-bit architectures for maximum compatibility.