PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.

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

DocuPUB - DocuPUB is an online file converter, file merger, file compressor, and file resizing tool.

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

PDFill - Open, View, Edit, Save and Print PDF Documents without Adobe Acrobat

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.