Finding good free synths always feels like scrapping the bottom of a barrel. There are all sorts of goos ones, but there is also a lot of crap. Here is a list of VSTI's I have used and loved. These are all for Windows Only.

Rez a Rad Monosynth

Rez is a seemingly basic synth, but it’s capable of a very wide range of sounds. While Rez has a bunch of cool features, the beating heart of Rez is its filter. It’s made from multiple chained filters acting as a single powerful filter and it’s highly resonant, providing Rez with its distinctive character. Top that off with distortion, a phaser, various oscillator modulations, delay, and arping.

From funky squishes, beastly basses, and screaming leads, to tight arpeggiations, swirling rhythms, atmospheres, bells, and interesting effects. You can download it at: http://www.ugoaudio.com/plug-ins/rez

Cosmogirl II

 It's a 2 oscillator analog style synth, specially  geared to the creation of spatial FXs,  ambient textures and experimental music. Here is a video with some exmples of  the sounds.

 You can download the synth at http://www.uv.es/ruizcan/p_vst.htm as well as  some of there other cool VSTi's.

 I mainly like there bells and find this a good way to add effects. Been supper stable  as well.

Alieno: the battle of sound begins

Alieno is a rad little synth. Cool looking GUI and good presets. Suitable for lead lines, sequences and special FX synthesis. SCI-FI look and feel with 12 skins and 144 original sounds extended library included.

Download free at: http://www.blank-media.it/acrobatics/

These are all great VSTi's that have been stable and great sounds for me in the past. Enjoy them!