TB Pro Audio produce two free plugins that I use on almost all my mixes, inserted on my Master Fader, to check against what my ears tell me as I edit my audio.
MvMeter is a metering plugin, download it here.
Isol8 is a free advanced mix monitoring tool. It divides the frequency range into 5 bands.
These 5 bands can be soloed or muted individually. This will help you to concentrate on certain frequency ranges during the mixing and mastering process. Download it here.