tweakbench has a bunch of really good VSTs and ive picked up a couple for myself. if you go on their site, you can pay for them. but i think thats just to support the developers, you can get them for free as well.
socalabs has a couple of VSTs as well, and they have a collection of synths that emulate sounds from certain videogame consoles like the gameboy or sega master system.
i'd also check out synth1, its extremely popular and for a good reason. it was originally hosted on geocities which is of course down, but you can find it other places.
there's also magical 8 bit plug, but i had a bit trouble finding it because it was on a japanese website + i dont think they support it anymore..?
i have a few friends who use LMMS, an open source DAW. i havent tried it out for myself, but ive heard good things about it. im not aware if it supports plugins (VSTs) though, but i may be wrong.
and of course, audacity. but everyone knows about that.