The Wikipedia article on Marxist dialectical materialism (always a good place to start!)

The “Di” entry on Marxists.org glossary of terms; “Dialectics” and “Dialectical materialism” are the first terms

The Learn Socialism facebook page, a series of very good and very helpful infographics

What Is Dialectical Materialism?

Kropotkin, The Conquest of Bread

Kropotkin, Mutual Aid: A Factor Of Evolution

Proudhon:  What Is Property?

Marxism-Leninism-Maoism basic course

Lenin, The State and Revolution

Trotsky, The ABCs of Materialist Dialectics