ExpDigital have announced Infinity API a free cross platform (OSX Universal Binary, Windows) toolkit written in C++. It’s split into four parts in order to manage specific tasks effectively.
ECore provides handling of all basics (such as container classes, object wrapping, host querying etc). Contains no graphical objects, but is all you need to write command line applications.
EGUI provides the graphics handling process space of the Infinity API. It features both leightweight controls and heavy weight window objects and is a consistent interface across both window and mac systems.
EMusic provides acess to musically relevant processing classes, such as Audio file I/O, Midi File I/O and event handling etc. This can be considered to be under construction - over time this library will grow!
EVst provides cross platform wrappers around the VST 2.3 specification and provides helper classes to save you time in both setting up and running your projects.