HISE is an open source audio framework for building virtual instruments and effects.
It emphasizes on sampling, but includes some basic synthesis features for making hybrid instruments. You can build patches, design a custom interface and compile them as VST / AU plugin or iOS app.
HISE is built around the JUCE framework. According to its description, the sampler features include Disk Streaming, cross fade looping, Sample Start Modulation and Round Robin group.
Check the list of features.
The license is GPL3 for free libraries, or commercial for commercial libraries. If you want to release a precompiled library (VSTi or AU), you'll need to purchase a JUCE license.