I've been using Cakewalk Sonar X3 Studio for a while now and for mixing purposes I have been using a combination of the free plugins that come with it and a whole load of freeware VST stuff (TDR, Molot/VladG, TB, Variety of Sound, Shuttlebug, etc, etc). While there is a wealth of excellent free plugins out there for, I've so far been unable to find a plugin compressor that has all of the options and functions of the Sonitus compressor, which has a lot of useful features.

It's a pretty decent all-purpose compressor. One can vary the ratio from 1:1 - 30:1 or Infinity:1, the attack time can go down to 0.0 seconds for fast limiting. It has a highly flexible hard/soft knee. Basically, it can meet almost any practical compression/limiting application. For some tasks, the actual sound of the compressor is not amazing, but I use it so often because it has these functions that the others do not.

The main thing that it has over all the other plugins I use for compression though, is that it can be used as a sidechain compressor.With the Sonitus, I can use sends from other channels as Sidechain inputs into the Compressor to trigger the compression/ducking/pumping - ie with kick/bass sounds. I can't get the option of sidechaining with any of the other compressors I use. At least not in the same way or that I am aware of.

Also regarding the Sonitus compressor, when it comes to compression on instruments for bass sounds (ie bass guitars and kick drums/toms), I prefer the way something like the TDR Feedback compressor sounds on the low end - fatter, rounder, warmer, more 3D. These tend to be the sort of instruments where sidechain processing is most useful for the kinds of music I work on. This is a slightly less pressing issue than the simple ability to sidechain, but given that it is these instruments and frequencies that I am mostly looking to use sidechaining on I thought it worth mentioning.

I know if might seem like a big ask, but can anyone recommend any (preferably free) VST compressors that have this function?

I know there are probably a number of top-flight plugins that cost varying amounts of money, but as a general rule of thumb I prefer to spend my money on actual physical hardware rather than software that is usually obsolete far more quickly. I've also stopped using cracks and ripped software. Of course I am willing to bend this rule if there is a great piece of software that meets my requirements.

Any suggestions appreciated.

Reaper has a free set of 32 and 64-bit VST plugins called ReaPlugs. Both the Gate and Compressor have side-chain inputs available on them.
 
Quick Edit: Not sure which version of X3 you have, but the PC4K Compressors for the ProChannel also have side-chain inputs on them.

I'm not at my DAW right now, and I may be wrong, but doesn't the Molot have an external side chain mode?

Some of Voxengo's comps have sidechains (Crunchessor, Deft Compressor).  Cakewalk's own (quite excellent) CA-2A does also.

I think most plugins need to be VST3 to offer their sidechain inputs to Sonar, or at least that has been my experience with them. Maybe I didn't look well enough before.
 
I don't have any free suggestions but FabFilter Pro C2 does everything you could want and more. It's not cheap though. 

Sanderxpander
I think most plugins need to be VST3 to offer their sidechain inputs to Sonar

My experience with plug-ins which have sidechain support is VST2 and VST3 work equally well. The exception is Waves which requires VST3 for their sidechain implementation.

Ah thanks for that, I do have a sizeable Waves collection and never had any sidechain inputs appear other than Sonitus until VST3 arrived. Go anecdotal evidence! :) 

Mettelus - I haven't tried the reaper plugins yet. I tried to downlad them a while ago but had a problem with the installation.

I have X3 studio, which strangely did not have any of the pro-channel compression until about a month ago when it asked me if I wanted to update. But I only get buss compression as an option on a few busses. If I add more busses, I only have the eq, saturation, and tube thing.

fjz - I haven't managed to find a way to sidechain using the Molot. So if it can be done, I'd be very interested to hear how. On the single channel version - the one that is just bars, numbers and letters (ie not the version that looks like the inside of a russian tank) I found a way to have an external sidechain input but couldn't get the paramaters working in a satisfactory manner.


Anyway, the ones I've tried that work for sidechaining for me (apart from the sonitus), are the following. The Variety of Sound plugins all require you to set the sidechain path as external once the compressor has been inserted.

The best two were:
- RedAmption Compressor - Straightforward compressor. Can do fast attack and release. Most useful one I've found so far.
- JB Red Phatt Pro - Really funky compressor that has loads of cool options. Also works for what I wanted it.

The others that worked for sidechain routing but were not that great for the task I had in mind:
- Thrillseeker LA - A little esoteric for straight-up pumping, and not enough control. Could work great for some things, but not as a general sidechain comp.
- Density MkIII - More of a buss compressor and there is not enough control over attack/release times for my liking as a sidechain compressor.
- NastyVCS - Channel strip. Not much control over the compression parameters.
- VC64 - Channel Strip, has a good compressor in it but is a bit overkill to me.

Gideon K
fjz - I haven't managed to find a way to sidechain using the Molot. So if it can be done, I'd be very interested to hear how. On the single channel version - the one that is just bars, numbers and letters (ie not the version that looks like the inside of a russian tank) I found a way to have an external sidechain input but couldn't get the paramaters working in a satisfactory manner.

Hi Gideon,
 
I`m not sure if you`ve tried this or not, but on the Molot GUI, there`s a little arrow at the bottom left side that can be easily missed.  If you click on that arrow, you will see some extra parameters that includes one where you can change the side chain from internal to external.  Also, I would make sure to have the latest version as this feature may have been added sometime after it`s initial release.
 
Hope this helps

There are some alternatives to compressors for sidechain
http://www.kickstart-plugin.com/
http://www.waves.com/plugins/oneknob-pumper#introducing-oneknob-pumper