Academia.edu – for researchers to share and follow research.
Mendeley – other than being a free reference manager, Mendeley also functions as an academic social network; acquired by Elsevier in spring 2013.
Social Science Research Network (SSRN) – a research network for dissemination of scholarly research in social sciences and humanities and is composed of a number of specialized research networks in each of the social sciences.
ResearchGate – a free research networking tool.
VIVO - An interdisciplinary network of scientists facilitating scholarly discovery. Institutions will participate in the network by installing VIVO, or by providing semantic web-compliant data to the network; developed at Cornell University in 2003 with an NIH grant.