Audio Vitamins' Contra lets you compare audio plugins on a channel strip - without using your mouse. The free version lets you load up to 3 Audio Unit plugins.
Compare Audio Plugins with Contra (Free)
From Audio Vitamins:
Contra is a plugin that is designed to help you make some crucial decisions in the mix without having to grab that mouse and click. Keeping your ears focused on the mix, will mean that your mix will sound better and you’ll get to the end product quicker. You will be able to choose between more options in a shorter time, letting you hear different plugin chains and compare them without having to click mute and unmute numerous times and forget what you like.
How it Works
Once you have scanned your plugins (this may take a while) and loaded up a few plugins into Contra, you can click on the orange settings icon to bring up the plugin's user interfaces. To compare plugins, set the numbers of bars you'd like to audition in for each plugin. This should match the number of bars you are cycling in your DAW. After pressing play, Contra will cycle through all plugins that are switched on.
See how Contra integrates into a Pro Tools workflow:
The free version of Contra lets you load up to 3 VST, VST3 or AU plugins, and you can compare your audio plugins in loops of 1 or 2 bars. The paid version costs £40, and lets you load up to 8 VST, VST3 or AU plugins. Plugins with settings can be saved as presets.
Download Contra Free (requires your name, email, and address).