DocumentWallet seems to be the latest in a new category of Mac applications, the PDF manager. Much like Yep (which we covered when it was called kip), DocumentWallet is an iPhoto-like application for keeping track of PDFs. In addition to its library functions, DocumentWallet allows you to scan document into its library with any TWAIN compliant scanner. One nice thing about DocumentWallet is that the PDFs are not stored in some sort of proprietary database, but in the file system (though the same is true of Yep). The maker of DocumentWallet also offers a very similar program called ReceiptWallet that does much the same thing, but focuses on saving receipts rather longer documents, and even includes some rudimentary math functions for keeping track of expenses. Like Yep, Yojimbo, and others, DocumentWallet also adds a convenient Save PDF to DocumentWallet option to the PDF pane of the OS X Print Dialog.
DocumentWallet and Yep are very similar, but there are some differences, perhaps most importantly Yep's focus on tags and DocumentWallet's focus on categories. Which one works better for you will probably depend on how you work, though both are worth a look.
DocumentWallet and ReceiptWallet are $29.95 each (or $44.93 for both) and a 21-day demo of each is available.