The web version of Mendeley is available at www.mendeley.com. Mendeley web allows you to access your Mendeley library and the social features of Mendeley away from your desktop, anywhere you have access to the internet. At present Mendeley web does not have all of the features of Mendeley desktop, but improvements are being made to bring the two platforms in line.
See Mendeley's guide to Mendeley web for more information.