JUCE (Jules' Utility Class Extensions) is an all-encompassing C++ class library for developing cross-platform applications. It's particularly good for creating highly-specialised user interfaces and for handling graphics and sound. The intended users are developers who are writing large, complex applications in C++ and who would like to use just one clean, high-level API rather than a bunch of different libraries for different purposes or platforms.
The Juce Audio Plugin Framework is a project for writing cross-platform audio plugins using a generic wrapper framework. The intention is to allow the same code-base to be recompiled into a range of different plugin formats, such as VST for the PC and Mac, AudioUnits, RTAS, etc. The code for this is also included in the Juce source tree.
JUCE also includes Juce Plug-in Host, a modular plug-in hosting environment, currently supporting VSTs. This is in the normal Juce source tree but there are some pre-compiled binaries available for Windows and Mac OS X.