Useful resources on the web
The works of Marx and Engels
Marxists.org Internet Archive: Marx and Engels, plus many other authors. Muiltiple languages.
From Marx to Mao: Marx, Engels, Lenin, Stalin, Mao, and some miscellaneous works
Christopher Caudwell writings (on marxists.org)
Race and Class - Important English language journal on racism and imperialism.
Rethinking Marxism - English language academic journal "to stimulate interest in and debate over the explanatory power and social consequences of Marxian economic, cultural, and social analysis."
Capitalism Nature Socialism - Journal of Marxism and ecology, nature, etc.
The Red Critique is an online journal of "Marxist theory and critique of the contemporary." It is published by the The Red Collective.
Institute of Race Relations - News and information resource on race and refugee issues in the UK and the rest of Europe. Publisher of Race and Class (in English)
resistir.info - Lots of content and links (in Portuguese)
For some interesting contemporary work on Value see the International Working Group on Value Theory site
The Institute for the Study of the Science of Society site has several sets of resources:
Institute-in-a-Box: An introduction to the science of society
Institute Resource Papers
The 1923 U.S. Army Field Service Regulations manual (15mb - big! - PDF file - requires Adobe Acrobat)