ORCID aims to solve the author/contributor name ambiguity problem in scholarly communications by creating a central registry of unique identifiers for individual researchers and an open and transparent linking mechanism between ORCID and other current author ID schemes. ORCID essentially does for authors what a DOI does for research papers.
ORCID is quickly becoming the default option for author identity, and publishers and funders are beginning to require that researchers have one. ORCID is also non-profit and independent of any major publisher or indexer (unlike ResearcherID).
ResearcherID provides a way to manage and share your professional information and solve author identity issues.
ResearchGate is promoted as the Facebook for scientists. It is a professional network to find collaborators, make your work more visible and stay current. It is free to join.
Academia.edu is a place to share and follow research, and see analytics on your profile and papers.
Mendeley is a very popular reference manager that also allows you to network and collaborate with colleagues. The basic version is a free download.
NOTE: These are commercial sites. This means that we cannot predict their long-term viability or business models. This has implications for the preservation of your papers and the privacy of your data. For these reasons, these are not appropriate repositories to archive copies of your manuscripts. And will not comply with your funders' open access policies.
See this blog post from the University of California, Office of Scholarly Communication, for a thorough discussion of this topic: