I'm aware this is a fair bit of necro going on, but I feel that my contribution makes it worth the bump. I have quite the selection of freeware VST's and VSTi's:
Variety of Sound (if downloads are down, look it up on vst4free.com)
These are the best plugins I got. All of them. Amazing compressors, chorus echo, reverb, tape simulators, saturators, exciter, have it all. No-bullshit, super powerful, mostly CPU-efficient, what more do you want.
Usage samples included! Oriented towards the rough edges of krautrock and experimental shit (the plugins are not bug-free), but applicable to everything.
If you want to get crazy creative and/or scientific. Linking to the WayBack Machine, because while the site is still up, I've not yet found downloads of the VST(i)'s on it, and the site has generally become impossible to browse, and all VST(i)'s are available on the WBM so. No clue what the author's intentions were. But yeah, highly recommend checking this out!
Rugged yet solid plugins that offer a lot of convenience and fun creativity. Some very neat experimental effects (like a "FSU" (fuck-shit-up) that "mind melts" audio based on text input) and some very efficient synths (like the tiniest GUI I've ever seen that produces simple sub basses or kicks).
Honestly I have only ever tried the Ambience plugin, but it is reason enough for this to be included, as it's the best reverb plugin in my possession, together with Variety of Sound's. I think that's enough said.
The most straight-forward, simple, basic and not-so-basic plugin package imaginable. Works really well. Also open-source!
Usage samples included! The things these plugins can do, you just don't see them anywhere else. Very impressive.
Be aware that DarkWare's quite a big name in the freeware plugin world, and it's been dead for a while. That's why the link is to the WayBack Machine archive of it, which fortunately includes all plugins' downloads. Anyways, highly experimental plugs, not bug-free, but occasionally very effective, especially if you want to get messy. I love THE BENDS, and GlitchGirl occasionally works really well at the beginning of an FX chain mixed in on a very low percentage (on anything really). DarkWare, when it stopped, continued under the name NOVUZEIT with commercialware, but that too got discontinued and disappeared completely, so sadly, that collection's become pirate-only.
Xen-Arts (original site went down when the dev lost interest dangit)
There's a lot wrong with these plugins, mostly in terms of intuitiveness, but for freeware, they're a magnificent option if you want to get your feet wet with microtonality! Some even let you reconfigure the entire harmonic series that's used to generate sound!
Very confusing site, but if you bear with that, some really useful tools come up. Some simple things that you don't see every day that could come in extremely handy!