PDF Clown - PDF Clown is an open-source general-purpose library for manipulating PDF documents through multiple abstraction layers, rigorously adhering to PDF 1.7 Specification
PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
Document .Net - Gives you API to do everything with a document in PDF, DOCX, DOC, RTF, HTML and Text formats.
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
PDFSharp - Office & Productivity and Development
PDFill - Open, View, Edit, Save and Print PDF Documents without Adobe Acrobat