Let me preface this with: I'm sure Howard Zinn, who died today, thought or wrote all sorts of perfectly awful things. And I haven't read him or thought about him much in decades, and could no longer meaningfully discuss his work without hours of refreshing my mind.
But I'll say this for the very popular historian among the progressive left: when I, as a youngish libertarian around age 19 or so, read his million-selling A People's History of the United States, I had zero problem seeing in it enormous amounts of education and intellectual ammunition for my then and now general view of government as a historical criminal.
I even wrote an article for my college paper's entertainment supplement on the 4th of July on a tragical history tour of U.S. government crimes that I recall was largely sourced from him. He helped remind me that when it comes to history, politics, and knowledge in general, it's foolish to think that only the ideologically sympatico have valuable things to teach.