ISOL8 by TBProAudio is a fantastic free multiband filter plugin designed to monitor your mix within a targeted range.
Once placed on your master or individual track, it will divide your mix into five separate bands. The solo and mute buttons then allow you to focus in on only the frequency bands that you are interested in. The cross-over range for each of these bands can easily be adjusted using the knobs at the top of the plugin.
The useful channel mode control can be changed to only apply the filter to the selected channel. Options include Left, Right, Mid/Sum, Side/Diff and Multichannel modes. The slope control can also be used to set the filter slope to either 24 or 48 dB/Oct.
In addition, the monitor control allows you to hear the isolated range on the Mid, Left, Right, or Side channel. The plugin also sports a handy DIM Button allows you to check your mix on a pre-defined lower volume, as well as a filter bypass switch and a master volume control.
I've found ISOL8 to be a hugely useful plugin, and after only a couple of days use it has already found its way onto my default Ableton Live template. The ability to target specific parts of your mix allows for some very precise monitoring and helps make detecting issues that much easier. It also looks absolutely gorgeous with an elegantly designed UI by Crimson Merry that is a real pleasure to use.
Apart from diagnosing issues with my own mixes, I found it quite interesting to use the plugin on professionally produced reference tracks to get more of an understanding how they get their mixes so clean sounding.
ISOL8 is available as a free download for all major plugin formats for both Windows and Mac from the TBProAudio website.
For more free VSTs check out this article.