A method.

Marxists have relied primarily upon the dialectic as formulated by Hegel and the post-Hegelian left. This has allowed them a certain broad understanding of history and its nuances, of the world as a process of change and transcient relationships. It functions as a ground for their systems of thought, an image of a process that can be applied in myriad contexts.

It inspired provocative extentions of Marxist theory into new terrains, political and intellectual. I'm personally impressed with their ability to use the dialectic to give confidence to workers engaged in union struggles and to articulate those struggles in class terms (their relationship to capital, the engines of material society) instead of simply ethnic terms.* In the intellectual sphere, it has been highly effective for mobilizing impressive ideas about development and learning (esp. with Vygotsky), evolutionary theory (Gould, Lewontin), and political economy.

This sort of concept, one expressing deep relationships animating change in the world, is useful if only because it functions as a ready tool for considering an array of situations.