Warning, you could easily spend weeks just D/Ling and trying these things out, don't forget about making music!
Crystal: A very cool synth with pads, ambient, and atmospheric presets plus much more
linPlug: Albino, Octopus, Cronox3, RMIV, SaxLab, daOrgan, ALPHAClassic RM III, FreeALPHA. All are fully functional demos which will make a noise about once a minute.
Cameleon: Another very cool synth. 800+ Presets, additive Synthesis, morphing, resynthesis... Demo
Sonik Synth: A limited version of a powerful synth. It has "rich ethereal and atmospheric moving textures but also fat analog flavors of nearly every classic synthesizer brand ever made! Imagine having the characteristic waves from vintage Arps, Moogs, Obies, Prophets, Jupiters, mega modular synths and more called up by a built-in search engine with pictures of the instruments for reference"
Zoyd: A virtual Synthesizer that has a rich feature set embedded in a semi-modular architecture to produce unique sonic experience.