polyIblit is a subtractive synth with alias-free oscillators, and sine, triangle, sawtooth and pulse waveforms. It’s a VST 2.3 plug-in for PC. Get it here.
2. Crystal Soft Synth
Crystal Soft Synth is both a subtractive synth and a frequency modulation synth with a semi-modular design. It features graphical editors, temp sync for envelopes and delays, and built-in effects such as chorus and echo. Get it here.
ASynth is a virtual analog synthesizer with two oscillators and three filters. It’s technically donationware, but if you can ignore a nag screen this synth is practically free. Check it out here.
Oatmeal is a simple subtractive synthesizer with two oscillators. Despite its simplicity there are plenty of parameters to tweak and it’s even skinnable, if you’re so inclined to tinker with your VST plug-ins visuals. Get it here, near the bottom of the page.
5. Ticky Clav
Ticky Clav is a virtual clavinet synthesizer—it recreates the famous clavinet sound you’ll know from Stevie Wonder’s Superstition. Though it does attempt to recreate the clavinet sound there’s plenty of room for tweaking and there’s an included wah-wah effect. Get it here.
Synth1’s web page may be unreadable unless you know Japanese, but that doesn’t mean it’s not a good, free choice. Synth1 features two oscillators, FM and ring modulation, four types of filters and distortion effects, among many other things. Check it out here.
7. Satyr VA Synth
Satyr VA is a virtual analog synth with two oscillators with FM and ring modulation, delay and two filters. The Satyr is the second plugin on the page—check out the screenshot to see this synth’s cool medieval design. Get it here.
Adonis is a four-waveform synth with ring and FM modulation and some built-in keyboard settings so using a MIDI controller is much easier. The Adonis has a built in arpeggiator too. Get it here.
How can you not try out a synth that looks like Alieno? Alieno is well-suited to lead lines and special effects as a monophonic synth with advanced step-modulation. Alieno features six oscillators, two multisaw generators and several built-in effects such as delay, vibrato and reverb. Get it here.
Xhip isn’t pretty, but it’s an analogue subtractive synth that’s smooth and punchy all at the same time. If you can handle working with the basic interface until a makeover is released, you should check it out here.
Helix is an aliasing-free synth with forty two modulation slots and advanced distortion controls. The whole synth is designed in stereo, so the sound is always big and wide. It’s considered one of the kings of free synths. Get it here.