• www.qrstuff.com ? You can generate QR Codes (the most popular 2D barcode) for Website URL, YouTube Video, Google Maps Location, Social Media, iTunes Link, Plain Text, Telephone Number, SMS Message, Email Address, Email Message, Contact Details (VCARD), Event (VCALENDAR), Wifi Login (Android Only), Paypal Buy Now Link. With QRstuff.com you to change colour of the positive QR Code shapes (the dark part).
• qrcode.littleidiot.be ? You can generate QR Codes for Regular text, Hyperlink, Phone number, Email address, meCard, vCard, WiFi settings. There is also the option to download the QR Code in multiple formats, particularly vector based formats. QR Stuff only does raster (bitmap) formats.

1. Tweet a promotion for a fee. Ex. “Just took dishes out of dishwasher and no spots. Thanks Finish http://bit.ly/Avpl3“
2. Background image real estate. Charge for a monthly rental, or even daily, for having your background image a flyer for an event or promoting a product.

The more followers you have the more you could charge.

What if there was a way to have a promotional line after an employees signature. So after:

Betsy Sue
Branch Manager
Company Ltd.
555.555.5555 ext. 555
123 Fake St.
Somewhere, Someplace
000 000

You had:

Our new optimax server is rich, see more.

And ‘see more’ went to some promotional page.
I wouldn’t see that as spamming or advertising, because you’re already dealing with the business. It’s just a simple way to promote.