Introduction to Marxism:
Marxism and Marxist
Socialism
https://www.marxists.org/subject/students/index.htm
Encyclopedia of Marxism (so you can look up any event, concept, author, historical figure...)
https://www.marxists.org/glossary/index.htm
Marxism on diverse subjects such as education, art, ethics, etc.:
https://www.marxists.org/subject/index.htm
Selected Marxists Archive:
https://www.marxists.org/archive/selected-marxists.htm
Bolshevik Authors:
https://www.marxists.org/subject/bolsheviks/index.htm
Main communist currents:
Left-Communism
https://www.marxists.org/subject/left-wing/index.htm
"Marxism-Leninism" (Stalinism)
Stalinism
https://www.marxists.org/subject/stalinism/index.htm
Trotskyism
Trotskyism
https://www.marxists.org/history/etol/index.htm
Maoism
Maoism (aka Mao Zedong Thought)
https://www.marxists.org/subject/china/
Syndicalism and Anarchism (in relation to Marxism)
Anarchism
Anarcho-Syndicalism
https://www.marxists.org/subject/anarchism/index.htm
Disclaimer: I think that these currents, while they are still somewhat useful at delineating one's political legacy or baggage (some of us reject them as labels or "identities" and might only concede to use them for the purpose of brevity in explaining one's tendency), they are partially outdated and as of now and little more than labels since most of these tendencies belong to a specific historical context with specific historical events and controversies and the main reason the left is so irrelevant today is the inability to move past them, the inability to transcend the XXth century and start focusing on the many fights and the open fronts we have
now. This is not to say that there is no discussion to be had about the events of the past century, just saying that it should not be our first priority. Even calling oneself a communist or an anti-capitalist is fucking meaningless nowadays, any idiot can do that. There still are social-chauvinists and anti-"cultural marxists" out there for fuck sake.