Well now, lets do this. I'll make a post in both k?

Helix
http://www.kvraudio.com/get/2709.html
Freaking awesome... its super complex, it eats CPU, but it's damn good. It has one of the best supersaws I've heard, almost as good as the real thing (JP-8000/SH-201).

Basic 64
http://www.kvraudio.com/get/2946.html
This is a great 8-bit synth. Its very powerful, and comes with an amazing group of presets. My only issue with it is its tuning, but that's easy to correct with a bit of tweaking.

mda DX10/JX10
http://www.kvraudio.com/get/209.html
http://www.kvraudio.com/get/42.html
Both are GUI-less, great for easy automation, and they are both great synths. They seem to have a warm, old skool feel, and I find myself using them in a lot of my songs, especially the FM E-Piano on the DX10. It sounds just like the DX7. The DX10 is great for people trying to learn about FM Synthesis... and it is, dare I say, better than Vivaldi MX. Also, they use a very small amount of CPU.

Silkworm
http://www.kvraudio.com/get/1800.html
Great synth for trance leads and strings, built in gate is nice! It does eat CPU though, so be careful.

String Theory
http://www.kvraudio.com/get/1074.html
Self-explanatory.

SynthString
http://www.kvraudio.com/get/1331.html
Great plugin for old skool sounding textures! It fits well in mixes, and the GUI is a joy to use! I recommend this one!

(Super) Spook Keys
http://www.kvraudio.com/get/2558.html
Excellent Theremin re-creation. Its really useful, and it doesn't eat a lot of CPU, even though the GUI is big.

Synth1
http://www.kvraudio.com/get/214.html
Synth1 is great for learning about synthesizers. Zircon of OCRemix wrote a bunch of tutorials using one of them. (http://www.ocremix.org/info/Zircon%27 s_ReMixing_Tips_Compendium/Part_3:_Sy nthesizer_Basics)

Voyager
http://www.kvraudio.com/get/1768.html
Great synth for pretty much anything... as long as you download the preset banks. Otherwise, you will be totall lost. (http://www.kvraudio.com/forum/viewtop ic.php?t=88748) has some presets, but I remember seeing a huge thread of them. I'l look around for them later, and possibly upload them somewhere.

Anyways, next up is an even bigger post in the free effects one. Not for an hour or two, though!

yeah i have lots of list!

I don't have many effects, I've got more instruments, so I'll add to what navij11 has already.
I got helix the other day, so I agree there that Helix is awesome.

DSK Strings
http://www.kvraudio.com/get/2859.html
It's no East-West/Edirol Orchestral, but it's the best free VST that I've found that can handle orchestral strings. It's a pretty big 'un, but it's definitely worth it if you're trying to do classical and you don't have large quantities of cash to burn... It has individual instruments, and sections, all as legato or pizz, to allow a broad range of sounds from the one VST.

DSK Brass
http://www.kvraudio.com/get/2479.html
Again, it's a large free Brass section VST, it has all the common brass section, as well as others, such as piccolo flute and an overall brass section. It also has some instruments with mutes and other brass effects. Great for classical, and also jazz.

DSK Mini drumZ
http://www.kvraudio.com/get/3031.html
This is a nifty drum machine, it emulates the samples found on about 20 different keyboards. Very useful if you want to diversify your drums from the regular samples.
I've just noticed they've got a larger, full version, DSK Drumz machineZ, looks like it's based on the same principle. I'll check it out and post again if it's any good. Link: http://www.kvraudio.com/get/3109.html

DSK Odisea
http://www.kvraudio.com/get/3030.html
A space/ambient VST with a small amount of useful presets. It has a variety of applications in the ambient/electronica style.

dmiHammer
http://www.kvraudio.com/get/255.html
This VST is a simple percussive instrument. It provides a decent marimba/xylophone type percussion, but most of the presets are too limited to have practical use. It may have its uses in some classical works or as a special feature in an electronica type song, but that's about it. Nothing special, but it's certainly not bad to use once in a while.

Mini DiZi
http://www.kvraudio.com/get/1811.html
Now this VST is brilliant. It's based off the sounds of a certain chinese wind instrument (I can't remember which one), and it has a rich vibe about it, and it's a worthy substitute for a flute. The application can vary from oriental to classical, to electronica, and, although it is quite simple, you don't really need to get technical with it, the main preset is all you need. There's a full version of this VST called "Chinee Winds", by KongAudio, and that one costs money, but this free little instrument is well worth having.

Mini ErHu
http://www.kvraudio.com/get/1885.html
This is also by KongAudio, and also a full version "Chinee ErHu" is made to emulate chinese string instruments. This one has a few presets which are useful. The main sound is a bowed string sound, whereas the other presets include a more plucked sound and a varied tone. It has a large application in oriental music, but the uses beyond that are less.

Oberon
http://www.kvraudio.com/get/312.html
Useful in various electronica genres, particularly in the bass, it has some decent presets and often a gritty texture.

Rock
http://www.kvraudio.com/get/1274.html
Now, this is one of my favourite VSTs. It's large, but it's packed with presets, probably about the same amount as string theory, and it's got a great application for electronic music.

String theory... has been covered.

Now... the tweakbench VSTs. I've got them all for reference, but I've found that some are much more useful than others.

Carrillon
http://www.tweakbench.com/carillon
This is a chimes VST. It has a very specific application, but it's good to have around in case you need that chime/bell sound one day.

Field
http://www.tweakbench.com/field
This VST is great. It's an ambient/background noise emulating VST, and it covers things from rain to the home, to the street, tokyo, school and the store. Things that could really bring out an ambient song. It's fun to mess with, but there's no way inside the VST to tweak the effects more than the volume.

Monomate
http://www.tweakbench.com/monomate
I find that this one works whatever you do with it. You might not want to go with the presets, but the randomise button usually results in something decent, else you can just tweak with the octave, wave, phse, distortion and other basic controls that are simple and easy to blend in order to get a decent synth/bass sound. Great for beginners.

Padawan
http://www.tweakbench.com/padawan
This is probably one of my favourite tweakbench VSTs. A variety of sounds from the 2 ocsillators, it works well in the higher register, and sounds great as a lead, harmony or just a bit of background atmosphere.

Papaya
http://www.tweakbench.com/papaya
Similar to Carillon, it's basically a lighter bell instrument.

Peach
http://www.tweakbench.com/peach
This is an 8-bit VST, with a whole lot of presets in the standard NES style. It's great for just grabbing it, picking a preset and from there, it's ready to go. Perfect for simple 8-bit tunes.

Ritual
http://www.tweakbench.com/ritual
This VST has some pretty good presets, mostly good for gritty basslines, but there's a variety, and it should prove useful, here and there.

Toad
http://www.tweakbench.com/toad
A basic 8-bit drum VST, this instrument contains various samples recognisable from some old nintendo games. It can work the basic 8-bit percussion as well as adding some effects to chiptune songs, to bring out some extra character.

Triforce
http://www.tweakbench.com/triforce
A great 8-bit VST which is based around the user's preference, with the ability to tweak the sound they want, it comes with a diversity that Peach doesn't offer.

The good thing about the tweakbench VSTs is that they're all very simple to use, a user friendly interface, so the range of sounds are great for the way you can just mess around with them and have it figured out in no time. Simplicity.

Vivaldi! MX
http://www.kvraudio.com/get/400.html
I haven't actually checked this VST out too thoroughly, but it does have a large range of presets, and of those that I've tested, they sound really decent. So I'd say that it's a good one to check out, and just mess about with.

That's pretty much all the VSTi's that I like. I'll have to have a look at the links navij has put up.