CyberAtlas is probably the best starting place for Internet demographics. They sift, analyze, compare and chart the results of other surveys, and provide links to all the source materials. Nice!
Donna L. Hoffman and Thomas P. Novak are two of the smartest people on the planet. They've devoted their energies to this sponsored project to determine the marketing implications of the net. Good analysis of all existing demographic studies.
John S. Quarterman's company that conducts Internet surveys and publishes the results in regular reports. The best statistical work done so far on determining the number of people online.
A voluntary survey of World Wide Web users conducted every six months by Georgia Tech's Graphics, Visualization, & Usability program (GVU). Prodigy participates in this survey. Good information about what people do online.
The famous TV ratings folks are working their magic online. This survey done in 1995 in conjunction with CommerceNet, a consortium of large companies interested in net demographics.