If you have ever needed to combine different PDFs into one larger PDF document, you need Combine PDFs. Combine PDFs is an excellent freeware application for the Mac that does exactly what it says: it helps you to combine different PDFs into one file. It doesn't do anything else. Just combine PDFs.
Why would you use it? Let me count the ways.
Well, I scan in my receipts for expense reimbursement and combine all of the different images into one file to make it easier on my CEO when he's reviewing receipts. I also use it when I've signed an NDA, for example, and want to take the different output files and merge them into one larger file.
It's very hard to get this process wrong with Combine PDFs. You simply drag the files (PDFs or picture files like JPEGs) onto the program, then rearrange them (if you need to do so), and hit "Merge Files." That's it.
Sometimes the best applications are single-function and super-easy to use. Combine PDFs is one such application. If you have a Mac, you should get Combine PDFs. It's not free as in open source, but it is free as in beer. (And if you're so inclined, you can get all the source code here.)