A list of all the militias in the US:

https://en.wikipedia.org/wiki/List_of_militia_organizations_...

While there are some on the leftist side, the vast majority of them have right-wing politics.

There is one caveat; Federal law enforcement routinely infiltrate groups that are mostly violent bluster, and creates agitation. This is entrapment, and is considered legal. However, it is very hard to ascertain which are really dangerous.