Mac users should go check out ReceiptWallet. It's a digital archive for receipts, it hooks right into OS X's TWAIN scanner support, so you can scan paper receipts straight into it. It also groks the system's PDF Services, so receipts from online purchases can be added through the print dialog.

ReceiptWallet is written by Scott Gruby, who some folks may remember from his NotifyMail utility several years ago. I worked down the hall from Scott for a while when we were both in the subscriber group at Qualcomm.

No plans for a Windows release. Suck it. :P