I like Brainworx DynEQ, but there is a free version of TDR Nova, or you can pick up Waves F6 in a sale. I use dynamic EQ on vocals more than anything else. You will often get very different frequencies from the same singer depending on the register and dynamics of their part. Inconsistent proximity to the mic can also change the low and low mids throughout a part. Most regular culprits are 300-500hz, where you want to keep that for quieter passages but they become overpowering when they open up, or high mids for the same reason. Every singer, performance and song is different though. Second most common use for dynamic EQ for me is bass guitar, to keep notes more even when some pop out more than others.
----------------------------------- R.I.P Old Blue 11/85-12/04
I will second the bass guitars. Even the VST versions (at least the ones I have) are exactly the same. The dynamics aren't compressed as standard, so a compressor then light use of dynamic EQ. Oh yeah TD Nova GE - it was on a deal, otherwise the standard one was fine. Plugin EQs and compressors are two a penny these days. A good one like that can be worth having.
Toneboosters. Only really because I had the previous version without dynamic eq and took the upgrade. Like it. Very quick to dial in what I want it to do. Never found it lacking enough to look elsewhere.
A glance at a SSL or Neve channel (or any commercially available channel strip) will show why and when dynamic EQ may be used. Which is to condition any audio input during tracking. Mostly. But your question is which one. And where do you put it in your recording chain? I use the Apollo system so I insert these components into my Apollo twin DSP. But less expensive options exist for sure. Many REAPER users can suggest for you. Try as many (of the free or low cost ones) as you can. Good luck.