SAVIHost is described as 'Make VST instrument plugins into standalone apps'. There are five alternatives to SAVIHost for Windows, Mac, Linux and BSD. The best alternative is Carla, which is both free and Open Source. Other great apps like SAVIHost are VSTHost (Free Personal), Blue Cat's PatchWork (Paid), NanoHost (Free) and Pedalboard 2 (Free, Open Source).
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.