Many people who consider themselves as marxists consider that the mode of production of a society determines its essential nature, however many are stuck with a world view that appears to be constrained by the period of domination of industrial capital and they do not see the mode of production of Free_software as a new mode of production.
Raoul Victor's text on Free Software and Market Relations is a summary of a discussion with a marxist who does not see the manner of production of free software as a kernel of a new society.
The Wikipedia article on Karl Marx is excellent and Oekonux is mentioned in the Contemporary_criticisms section.
The Marxists internet archive has a large collection of the works of Karl Marx & Fredrick Engels, also under the GNU_Free_Documentation_License, in addition this site is mirrored on this server.