PDFmerge includes a complete user interface for merging/assembling multiple documents into a single PDF document for Mac OS X. You can add any number of documents (any number of times), choose a page range from each, order them appropriately, and then Merge them into a single PDF document.
In addition to existing PDF documents, other formats are supported that will be first converted to PDF and then merged and enhanced. On Mac OS X 10.2 (Jaguar), PDFmerge supports a number of raster image formats (PNG, JPEG, TIFF, PSD (Photoshop), etc.) as provided by QuickTime. When using Panther (Mac OS X 10.3), Postscript (.ps) & Encapsulated Postscript (.eps) documents are also supported.
Priced at $89, PDFmerge is shipping now for Mac OS 10.2 and up. You can download a fully functional demo version to try out on your own files.