Some of the more well-known VSTs in the world of guitar would be Guitar Rig, AmpliTube, and POD Farm. The programs I mentioned are a little bit different than most VSTs as they actually can be opened in standalone software, while most VSTs have to be opened in a DAW that supports the VST format. The list of DAWs that support VST would be a very long list, but a few examples would be Steinberg's Cubase (Steinberg actually invented the VST in 1996) and Cockos Reaper. The invention and implementation of VSTs have made it possible for unsigned bands without major label support to release studio quality, or close to studio quality, releases. It has also helped bedroom guitarists record their own songs, ideas, and covers.
VSTs primarily fall into a couple of types, VST effects and VSTi's (or VST instrument). VST effects would be virtual choruses, flangers, distortions, fuzzes, delays, autowahs, phasers, EQ, compressors, etc., while VSTi's would be virtual instruments such as synthesizers, pianos, orchestral instruments, guitars, drums, etc., that read MIDI input and then perform that input. There are freeware and commercial versions of most of these products, with wildly different levels of quality and performance from product to product. Some of these VSTi's work as standalone applications and can be used for practicing without an amp, or practicing keyboard, piano and synthesizer with a MIDI workstation. I'll try to give a commercial and freeware version of most of the most common VST plugins you'll need to get started.
First things first, and you'll need a collection of mixing/mastering effects such as EQ, Reverb, Stereo Imaging, etc. Most DAWs come with their own VST plugins for the basics, but I haven't personally come across a DAW that has the most desirable or user friendly VST plugins as part of their package deal. One of the most popular commercial programs for VST, and a solid product is iZotope Ozone. This is currently on version 6, and includes a Maximizer, Equalizer, Multiband Dynamics, Multiband Stereo Imaging, Post Equalizer, Multiband Harmonic Exciter, Reverb, and Dithering. There is an "advanced" version that offers even more. The price tag is hefty until you start adding up the studio equipment this would be replacing - it goes for $249 USD. iZotope Ozone also has a standalone interface.
For a "freeware" substitute, I use several products from Blue Cat Audio that are available in their "freeware pack." The products they offer for free are Triple EQ, Blue Cat Flanger, FreqAnalyst, Gain Suite, Chorus and Phaser and are available here. They are not offered as a standalone interface, but can be opened through a fairly intuitive interface in whichever DAW you use. Blue Cat Audio's free VST plugins are more geared towards effects than mixing/mastering, but I manage to accomplish most of what I need to accomplish with the Triple EQ, FreqAnalyst, and then I use a few different reverbs. I've mainly used TAL-Reverb-II and TAL-Reverb-III lately, and both are freeware available here.
Here is a video showing how powerful some of the freeware VSTs can be with Blue Cat's Triple EQ:
And here is a short demo of the TAL-Reverb-2:
There are dozens of sites that compile lists of freeware VSTs, some are even user-generated, and some of them are exceptionally good products. The user interfaces on VSTs are sometimes more intuitive than others, which means you have to find something you are comfortable working with.
There are also a lot of virtual amplifiers and cabinets available for guitar and bass. The more popular commercial VSTs for this are pretty much "household names" in this community. I mentioned them in the beginning of the article - Guitar Rig, AmpliTube and POD Farm. There are other commercial VST amps that are very worthwhile, as well, along with a few freeware ones if you take the time to look.
A majority of freeware VST amps and cabinets tend to be a "one trick pony," with a different VST needed for different types of tones. There are a few, however, that seem to do a pretty solid job across the board. I'll tell you what I use for my purposes, but there are very possibly newer and better VSTs out there. Two of the better VST amps would have to be Grind Machine Free, which is made by Audio Assault and includes 2 different amp models as well as some good cabinet simulation. I personally use this one in conjunction with other VST effects to get heavier tones. Audio Assault also makes a bass amp - BassGrinder Free - but I haven't used it. They make commercial versions as well as these limited-featured freeware versions. Another VST amp that I haven't used as much as Grind Machine Free, but have had a few promising sessions for cleaner and overdriven tones is the Ace Amp by Shattered Glass Audio. As far as a VST amp for bass, I would have to recommend the SHB-1 by Ignite Amps, which is essentially modeling a bass tube amp. 3rd Bass is another popular bass amp VST that is also free and has quite a few built-in effects.
Audio Assault Grind Machine
Shattered Glass Audio Ace Amp
Ignite Amps SHB-1
What about VSTi's?The world of VSTi's is a pretty convoluted place, and these jokers can get super expensive on the commercial side, and can get huge in actual size on your hard drive and very needy on computer resources. As an example of what I'm talking about, one of the better commercial VSTi bundles would probably be Native Instruments Komplete 10 (available for $499 USD), which contains 39 instruments and takes up approximately 130 GB of hard disk space. Even if you're trying to keep it simple, you may still need to use a VSTi for drum parts (in cases where you don't have a drummer, or you just don't have a good set of drum mics). A popular program, Toontrack's Superior Drummer 2 ($299 USD), takes up about 25 GB storage space for the drums alone.
Native Instruments Komplete 10, there isn't a good full demo because the program is SO massive and full featured, but here is a brief overview:
Toontrack Superior Drummer 2.0
The good thing about the commercial VSTi plugins, especially for drums and percussion, is they usually come with good high quality samples for each instrument, drum and kit. Many of the freeware VSTi drum plugins have lower quality samples, or no native samples included. This means you will have to take poorly done samples and try to make them sound good with creative EQing and mixing, or find freeware drum samples on the vast Internet and map them correctly with the VSTi, or - more realistically in most cases - use a limited free demo of a commercial product until you find the one you want to actually invest in, and then purchase it. There are a lot of boring but very useful videos available on YouTube explaining how to use freeware or commercial VSTi drums - you'll notice that usually the freeware videos are much longer because there are considerably more steps to make them sound good, usually.
Getting into the world of other (non-drum) VSTi's, some of my favorite freeware ones would be those made by DSK Music, which offers 53 separate VSTi's, many of which have multiple instruments, including some world and ethnic instruments that are otherwise very hard to find. Their interfaces are pretty simple, but with some tweaking you can usually get a pretty realistic end result. Here are a few samples of some of my favorite VSTi's made by DSK Music, and I strongly suggest checking out more of their freeware VSTs, or their only commercial product - DSK HQ Instruments package ($25 USD):
The DSK Asian Dreamz
The DSK Virtuoso
DSK HQ Instruments:
They are definitely worth exploring, but there are literally hundreds or even thousands of freeware VSTi's out there, and some of them are exceptional while some are not so great. I definitely recommend exploring some of the options available, but you've got to go in realizing there is a real time investment to finding the best match for what you're trying to do, and be prepared to turn some knobs before you find the sound you're looking for.
A quick word about using VSTi's - you've gotta be able to either manually program the MIDI, or use a MIDI workstation (keyboard) to actually play the notes, or sometimes a combination of the two. What I personally prefer to do is play it on guitar, then transcribe it into GuitarPro 6 as either guitar tabs or standard notation, then export it as MIDI, then open it in my DAW and feed it into the VSTi. If you are comfortable with a keyboard/piano, then using a MIDI workstation is definitely going to be preferable. After playing and recording the notes as MIDI, in most DAWs you can go into the MIDI information and fix the timing, mess around with the dynamics, or with most DAWs "humanize" your MIDI information - these are just a few tactics when working with VSTi's.
In ConclusionNow, just as a fair warning, there are a LOT of VST plugins, both commercial and freeware. In my experience, the commercial VST plugins aren't always higher quality or better than the freeware. Let me stress again, though, that there are a LOT of VST plugins available - like this is a whole world you can get lost in. And there are some really strange VSTs available. I mean strange like it will leave you scratching your head trying to figure out who uses these things, and what kind of "music" are they using them to make. To close out the article, I'll point you to a couple samples of some of the weirder plugins I've ran across.
First, you've got the Delay Lama by Audionerdz, a VST synth:
Alien Artifact is a VST synth made by Hercs Music Systems: