Guitarhacker
Ozone is my go to plug to sweeten and polish the mix a bit.
I also use Izotope's Ozone 7. If you can afford it, it's great for people who don't know too much about mastering, as it comes with dozens of presets (that you can tweak), and offer a wide range of presets, anything from very subtle, to full out "loudness wars" presets.
I've created a few custom presets that I use all the time. And it really adds that commercial pop to my songs, without being too loud. The individual modules in Ozone, offer an infinite amount of tweaking, and it's a great way to learn what to use when mastering. The UI is beautiful as well.
