Should have put this one up already; http://marxists.org/
All the major works of most major 'Marxist/Socialist' theorists online on a single site. The sheer volume of labour involved in putting this site together is staggering and it was all apparently done by hundreds of volunteers. The layout is very manageable too - almost the entire corpus of Marx's writings can be accessed in a flash + the complete works of Luxembourg, Kautsky, the Frankfurt School and hundreds of other key figures.
Even 20 yrs ago this would have been considered preposterous science fiction. Research is truly emerging from the Dark Ages.