Just to extend the question a bit, but is there a particular reason why these armed groups are called "militias"? After all, a militia is seen as a regularly organised quasi governmental organisation, whereas these groups seem to be anything but! Using the word "militia" seems to be giving them something of a official, regular, positive standing, suggesting that they are an inherently "good" thing.
While the tradition of anti-Government, armed groups typically tends to the right in American history, there have been several groups on the far left who have also resorted to violence in attempting to achieve their political goals. As you stated, I believe the Black Panthers would have classified into this group along with a few others. The Black Panthers, known for their activism, Maoism, and armed nature, became the Black Liberation Army in the 1970's and advocated for open, armed struggle against forces of racism, sexism, capitalism and imperialism. The Black Liberation Army killed police officers, and robbed and bombed banks before their dissolution in the early 1980's. The other major leftist, armed political group that comes to mind is the Weather Underground Organization (WUO) The WUO was a radical left organization inspired by the Anti-Vietnam rhetoric of the Students for a Democratic Society and the anti-racist actions of the Black Panther Party. The WUO undertook a series of bombings against government buildings, armored car robbery and general mayhem in the late 60's and 1970's. These groups were inspired by, and wanted to add to, the massive social changes occurring in the United States at that time caused in part by Vietnam and the Civil Rights Movement.
Also somewhat in this spectrum, and later on down the line, is the presence of leftist Eco-Terrorists such as Earth First! and the Earth Liberation Front who use guerrilla warfare and sabotage to protect the environment, destroying logging and mining equipment and spiking trees so that they can't be used for lumber. Though I can't speak much to these groups because they tend to use Lone Wolf style attacks and are fairly new (peaking in membership and action in the 1990s and 2000s)
As far as readings go i'd suggest Rage on the Right: The American Militia Movement from Ruby Ridge to Homeland Security by Lane Crothers and Bringing the War Home: The Weather Underground, the Red Army Faction, and Revolutionary Violence in the Sixties and Seventies. by Jeremy Varon