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.
A powerful sampler with Disk Streaming, native multi mic support, cross fade looping, Sample Start Modulation and Round Robin groups.
Combine multiple sound generators which are capable of the most basic types of synthesis: additive synthesis, subtractive synthesis, FM synthesis, wavetable synthesis
Use Modulators like LFOs or Envelopes to change dedicated parameters (Volume, Pitch, etc.) of other modules. You can even use Modulators to modulate the parameter of another Modulator that modulates another Modulator...
Use the internal Macro Control System to expose only some of your internal parameters to the end user and hide the complexity of your patch. Use the WYSIWG editor to build a user interface that controls these parameters. Export the instrument as package and load it in the HISE Player.