PDFsam (split and merge) is an easy-to-use tool to merge and split PDF documents. Console and GUI versions are available. The GUI is written in Java Swing and it provides functions to select files and set options. It's made over the iText library.
Merge PDF files, select the pages, merge bookmarks and interactive forms
Split a PDF file at given page numbers, at given bookmarks level or in files of a given size
Extract pages from PDF files
Rotate PDF files, every page or just the selected pages
Merge PDF files together taking pages alternatively from one and the other. The perfect tool if you have a single-sided scanner