iZotope
Ozone is great for beginners and advanced users since you have so many choices and you have the Master Assistant that will create a unique starting point for you based on your program material.
Plugin Alliance
Masterdesk is absolutely amazing. I have yet to hear something that didn't immediately sound better being run through it. It's simple to use for anyone at any skill level.
Waves has a variety of plugins for mastering that work well, like TG Mastering Chain.
You can demo all of these for free.