I will tell you right now - besides buying a Waldorf blofield or Virus Access variety hardware va...you are going to have an extremely hard time finding the kinds of sounds you will get from reFX's Nexus. I know that it costs a hefty $300 but it is definitely worth it unless you want to spend ages trying to replicate the presets that you will immediately have in front of you with that softsynth. Other extraordinarily good software synths for the price are Vember Audio's Surge, Lennar Digital's Sylenth 1 and GForce's Minimonsta. Cakewalk's Z3ta+ is also good for a little bit lesser price.
Freeware-wise...there are a few I always recommend to people:
Synth 1, Oatmeal, Atlantis, Minimogue and Arppe2600, the 2-voice H.G. Fortune synths, and Crystal.
You can google any of those with the word "vsti" added to it if you have trouble finding them - it should do the trick.