I launched a new free service today built with open source tools. It is a simple application that allows you to load multiple PDF files and then “remix” the pages from them in any order you wish.
When you are done remixing, you can download a new PDF file that contains the pages you selected in the order you defined.
This allows you to quickly combine a cover letter with another document, or remove some pages from a document before email it etc.
The site is pdfremix.com.