Adding tube pre-amp or running a sound through a cab can really add unique character to a synth. It’s actually hard to find really good VST’s that hold true to such hardware effects. The people at igniteamps actually made a whole host of awesome and free VST’s.
Nrr-1 Custom 3 channels tube preamp is an amazing effect to add to leads or dirty up your sound. They also have some guitar pedals. overdrive, tube amps, and more for free at there site. These are also for Mac as well as windows (a rarity for free plugins).