Sound and performance
If you search for this today, you are walking into a trap.
: A free parallel dynamic equalizer that can help achieve the "expensive" high-frequency sheen the 737 is known for. Voxengo Tube Amp
That being said, there are some reputable sources that offer free trials or demos of the Avalon VT-737 plugin. These demos often have limitations, such as restricted functionality or time-limited usage, but they can still provide a good taste of the plugin's capabilities.
There is currently no official, legal "free download" of the Avalon VT-737 software. The official hardware unit does not come with a software plugin counterpart, and major developers (like Universal Audio or Waves) sell their emulations as paid products.
You can recreate the VT-737 "channel strip" feel by combining these free, reputable plugins: