I was surprised to find out just how many years ago it was that I first checked out DDMF's Metaplugin — my review appeared way back in SOS May 2011. Since then, this most handy of problem-solving plug-ins has matured considerably. Not only is the interface more user-friendly, but this VST/AU/AAX/RTAS plug-in (for Mac/Windows) is now capable of hosting VST2, VST3 and AU plug-ins, bit-bridging, and up to 4x oversampling for the whole plug-in chain being hosted. It can also deliver DAW automation data to any parameter of any of the hosted plug-ins, and supports DAW plug-in delay compensation. And what's more, Metaplugin itself isn't all you get, because bundled with it are three other utility plug-ins that can be used either in your DAW's insert slots or within Metaplugin: the M-S encoder/decoder and multiband splitter came with the original version, but there's also now an audio routing plug-in called SendIt — and for users of some DAWs that last plug-in alone could prove worth the price of admission.
Metaplugin itself is essentially a plug-in wrapper that provides a blank canvas in which you can organise and route your own third-party plug-ins freely. By default, the GUI displays the audio and MIDI inputs to Metaplugin and outputs from it. It's your job to add plug-ins and link them together. You can load plug-ins directly, drag them from Finder (and presumably likewise from Explorer on Windows... I tested Metaplugin on a 2018 MacBook Pro running Mojave), or automatically scan chosen folders to populate a plug-in browser in Metaplugin itself. You can also right-click to insert a plug-in, duplicate plug-ins, and so forth. Plug-ins can be placed anywhere on the main section of the Metaplugin GUI, and the hosted plug-in's inputs and outputs are made visible graphically. Creating a signal chain is a simple matter of dragging and dropping from the Metaplugin inputs to the inputs of your plug-ins, dragging from one plug-in's outputs to the inputs of the next plug-in in your chain, and from the last plug-in to Metaplugin's outputs. The links between plug-ins are indicated by graceful, curved lines — and when you hover over one of those lines, you'll find a control to boost or attenuate the signal level.
It's all very intuitive, and creating complex effects chains (eg. with feedback loops, parallel processing and more besides) is a doddle, and you can save these as patches in your DAW. Cubase/Nuendo users in particular might be interested to note that Metaplugin will reveal the external side-chain inputs of any VST2.4 plug-ins that have them. And coupled with SendIt (of which more below) that can grant you access to side-chaining with VST2.4 compressors and gates — Cubase normally requires VST3 plug-ins for side-chaining.
Cubase users might be interested to note that Metaplugin... coupled with SendIt... can grant you access to side‑chaining with VST2.4 compressors and gates.
Slightly more complex is the parameter automation, and that's because the host DAW obviously can't directly 'see' the plug-ins hosted in Metaplugin. DDMF get around this by providing an assignable list of parameters — you map whichever ones you want to whichever parameter you wish to automate. This is obviously slightly more fiddly than accessing parameters directly in the DAW, but it's a sensible approach, and if you only want to automate, say, three parameters in a chain you won't have to wade through them all in your host DAW.
So what of SendIt? Well, this is reminiscent of the old freeware 32-bit Senderella plug-in that I used to use as a problem solver. You require two instances, one to act as a sender and one as a receiver. The sender instance can be placed anywhere where there's an insert slot — within Metaplugin, directly in your DAW, or even in another piece of software entirely. To return to that Cubase example, you could place the sender in your kick track's post-fader insert slot, and have the receiver feed the external side-chain input of a VST2.4 compressor inside Metaplugin to duck your bass part. When you stop to think about it, it could be useful in any DAW, though, as I can't think of a DAW that allows you to send from any point in the signal chain (such as from between two insert processors). You could, for example, use a send instance as a meter tap, moving the sender instance to get a reading on your meter from any point in your DAW.
I did make one suggestion to the developer that was warmly received, so I have high hopes that it will be implemented in the not-too-distant future: SendIt currently features a Thru tickbox that allows audio to pass through the plug-in as well as being sent elsewhere. My idea was to add a blend control, to balance the thru and sent signals — it would make it possible to create a 'BBC echo mixture'–style send (see here for details: http://sosm.ag/send-and-blend) very easily in any DAW.
Some of the other uses of Metaplugin are obvious. You can use it as a wrapper, for example to host VST plug-ins in Logic, or VST/AU in Pro Tools. It can also act as a bit-bridger, breathing life into 32-bit plug-ins (if your OS allows them). But really, the most fun is to be had when you start exploring what can be done with the ridiculously flexible routing inside Metaplugin; your imagination really is the only limit.