What are some alternatives?
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.
FreeMind - FreeMind is a premier free mind-mapping software written in Java.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
EndNote - EndNote is the industry standard when it comes to reference management in a comprehensive format.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.