The IK Syntronik VST instrument also has a Free version with 17 synthesizers and over 50 preset sounds in it, but you can also edit them and create your own sounds.

Native Instruments is one of the leading creators of VST instruments and effects. They give away 50 fantastic sounds.

PolyGAS is a synthesizer based on granular synthesis which basically uses small bits of a sample to create a new sound, and you can load your own samples to synthesize them.