Voxengo Boogex free
This thing is incredibly versatile and will go from transparent and super-clean to growly and snarly. I mostly use it as a bass amp sim, usually running the signal in parallel with a clean DI track. Sounds very "real" and snappy, gives life to the mids of the bass track like few other plugs will.
The included IRs are very handy too! The second one in, a beta52 4x12 impulse, is my go-to for quick and dirty bass sounds now. Sounds rich and smooth with just enough top end to pop through the mix without being too obtrusive.
The fact that this is free is hard to believe. Take my money!!!
It's pretty simple why Voxengo plugins are so good - Alexsey has a better ear compared to other developers.
His plugins sound so 'musical'. I've used many other plugins that technically perform their designed function, yet somehow just don't sound good.
I now have a whole set of expensive plugins that are just taking up drive space. They simply don't sound as good as thier Voxengo counterparts.