Many users here feel Izotope Ozone is one tool where you don't want to try the demo unless you are ready to buy it. After trying it, you will want it.


If I somehow lost every record of my copy of Ozone .. I'd go out and buy it again quickly and hope I could work it out with Izotope later, if needed. Yep, I'd risk buying it twice as opposed to not having it for a period of time.
That's about the closest *I* can come to a 'must have' recommendation.
Ozone is different than the above mentioned Izotope program, but after that you asked about 'mixing and mastering' must haves.
TBH I've used Ozone at both the track level and the mastering slot. Each instance would have much different settings, but it can be used in both ways, though it is most known for mastering.

I do not work here, but the benefits are still awesome
Make your sound your own!