Carla is an audio plugin host, with support for many audio drivers and plugin formats. It has some nice features like automation of parameters via MIDI CC (and send output back as MIDI too) and full OSC control.
NanoHost is not a synthesizer. It is a free, minimalist host application for Windows that can load VST-synths. Furthermore, it can act as a wrapper which converts VST-synthesizers into standalone applications.
A VST (plus AudioUnit on OSX) plugin host designed for live use. Based on the JUCE 'audio plugin host' code, heavily modified to better suit my purposes.
Blue Cat's PatchWork is a universal plug-ins patchbay that can host up to 64 VST, VST3, Audio Unit or built-in plug-ins into any Digital Audio Workstation (DAW) in one instance.This app doesn't have any features at the moment.