Digital Postman Pro is a straightforward program that has one simple job and does it. It allows you to put a document on your scanner (or load one from your hard disk), and then e-mail it. While the hard disk use is of secondary utility--it's as much work to use the $30 Digital Postman as it is to use your mailer, so why bother?--the scan-and-mail function is a serious time saver.
Digital Postman's interface is a bit on the unattractive side, but it has all the functions you need and they're clear. Scan, send. Very straightforward.
This program is not free, however, and you should only pay for a one-function program if you use that function a lot. If you're regularly scanning documents only to send the scans off, not to archive them, Digital Postman Pro is probably worth the price for the convenience factor alone.