Le Piano is a simple one sound piano module witch gives a Piano sound. Developed by Safwan Matni.

CS CLOUD SYNTHESIZER BY COLOR SPACE

Dead Duck Deducktion, a freeware virtual analogue synthesizer in VST plugin format

Gold Piano is a virtual piano for Windows available for free download.

Amazonia 5 is a freaked out & heavyweight software synthesizer.

Arppe2600va  is a free Virtual Instrument Audio Plugin for Windows.

Chamber Orchestra keys VSCO 2 Community Edition (CE) is an open-source, open-ended subset of the main VSCO 2 library designed for young composers.

Chamber Orchestra (Strings) VSCO 2 Community Edition (CE) is an open-source, open-ended subset of the main VSCO 2 library designed for young composers. Available for free download.

Daniel aka The Interruptor. BIONIC SUPA DELAY. This is an improved version of the popular "Bionic Delay" (Donationware)

The FB-3300 is a VST software instrument for Microsoft Windows, it is freeware virtual analog synthesizer

Fury-800 emulates the Korg Poly-800 hardware synth available for free download. 

Cosmic Boy Icecream · 8-bit/Chip style Mono/Poly synthesizer Available for free download

If you are looking for Gongs Chau Gongs VST by Alan ViSTa are great and a free download.

Chip32 is simple wave table synthesizer. Wave table is only 8bit

Ample Guitar M Lite II is a Acoustic guitar plugin developed by Ample Sound, that can be downloaded at no cost.

Chimera is a popular noise-driven synth and a free VST synth by Majken.

Arminator 2

A1TriggerGate is a ‘Sequenced Gate’ plugin to chop up any incoming audio signal. It is a well known effect in electronic music and sometimes also known as ‘Trance Gate’ effect.

Azurite by Distorque is a Virtual Effect Audio Plugin for Windows. It functions as a VST Plugin.

Dexed is a multi platform, multi format plugin synth that is closely modeled on the Yamaha DX7. Dexed is also a midi cartridge librarian/manager for the DX7. Fully works on Windows 7 and Windows 10. The Yamaha DX7 was originally used by artists including A-ha, Kenny Loggins, Kool & the Gang, Whitney Houston, Chicago and Phil Collins.