This isn't any article in particular, but rather a fantastic resource for anyone who wants to research leftist theory, or improve their praxis.
Disregard the name as it is a wealth of knowledge for socialists, anarchists and communists alike. I'm sure many here have learnt about this free and open library in the past, but just in case it's the first some have heard of it, I thought I'd share.
The Conquest of Bread — Peter Kropotkin (1892): the ideas hold up, but the examples are quite dated.
Mutual Aid: A Factor of Evolution — Peter Kropotkin (1891): was very influential over modern sociology, but of course newer research on this subject exists.