Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.

Aigaion Reference Management System - Aigaion is a bibliography management system that seems to be the perfect tool for managing your bibliography and references.

Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.

BibBase - BibBase.org is an easy way to set up and maintain a scientific publications page.

Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.

ProCite - ProCite is a tool for creating citations according to the standard defined by the users.