I pretty much stick with in the box plugs for mixing, pro channel stuff is indispensable, the compressors are great and the console emulations are really nice too, along with the quad eq, you have everything you need. I also make heavy use of the VX64 vocal strip. And I like the tape emulation so much that I started a love thread for it. :) And between Dimension Pro, Rapture and Z3ta and Addictive Drums, and all the others that come with Platinum, I'm pretty set for VST instruments.
For mastering I use Izotope's Ozone 7 Advanced, which I really like. To be honest, though, you don't really need it if you dig the sounds of the Nomad Blue Tube suite's stuff. I use the limiter for drum and bass buses sometimes, and I use the Audio TrackBox plug quite a lot for goosing up various instruments.
So I haven't felt the need to buy anything else to use for recording and mixing. Everything is there that I need right now, but I'm not a professional and I only record myself one channel at a time, so you should pay more attention to a real pro when one chimes in here.