First I just want to say there are no rules here so don't get me wrong. Use what ever you like to get the ultimate sound.
After reading many of your post here it seems that some of you mix up what a traditional word like--->
"2BusMIX" (MASTER OUT of DAW or Hardware Console) ..........VS
Mastering TOOLS means. That's two different cup of tee.
Mastering Tools are tools like Loudness Maximizing limiting and 2-Bus you often use more like a SSL Master Bus compressor. You can use MASTERING TOOLS on the "2BusMix" too if you like, as I said there are no rules what you can do or can't do but often you want to use that in a later stage or send your mix to mastering house.
Any how here is some of the
greatest SOFTWARE MASTERING TOOLS I would strongly recommend you to check out.
Nomad Factory Analog Mastering Tools (real nice warm musical Mastering tools)
http://www.nomadfactory.com/products/amt/index.html
iZotope Ozone 5 Advanced. "World class mastering"
http://www.izotope.com/products/audio/ozone/
Waves L3-16. http://www.waves.com/content.aspx?id=3502