You might not think about them much. When you do, it might be to decry the invasion of robot vomit onto every ad, billboard and poster. Still, the humble two-dimensional QR Code -- originally developed by a Toyota subsidiary to assist in automotive manufacturing -- has become ubiquitous.
QR Codes can deliver almost any kind of digital data, as long as it fits within the code's capacity; the densest versions of the barcodes can hold more than 4,000 ASCII characters, but most of the codes you see in public are much less info-packed. They're ideal for short URLs, vCard contact info, SMS or phone call "triggers" for mobile phones, and plenty of other cool tricks. I've used them on name badges for customized scoring forms and on stickers in out-of-the-way corners as scavenger hunt targets.
Unfortunately, some marketers seem to think that putting the codes on highway billboards (where you can't really pull out your phone to scan them as you're driving by at 55 MPH) or on subway platforms (the underground thing, not so much with the 3G data) is helpful and clever -- it's not. You need to put them where people have the luxury of time and the benefit of bandwidth; otherwise they're just clutter.
Although iOS devices don't natively handle QR Codes yet, there are plenty of third-party apps to enable code scanning. Over at our sibling site TechCrunch, Brenden Mulligan suggests that the best way to help QR Codes make the final leap from curiosity to mainstream tech would be to have the built-in Camera apps on iPhone and Android optionally set to "always looking for QR Code" mode while running. If there's a code in the field of view, says Mulligan, the app should prompt you to process the code. I can think of a couple of ways this would be annoying (what if you're actually trying to take a picture of an advertisement, versus scanning the code?) and potentially harmful (QR Codes can point to malware sites, so automatically decoding them isn't always the best plan). Still, integrating QR scanning capability into the built-in apps is a natural next step if handled gracefully.
On the Mac, however, you don't often think of scanning barcodes unless you're cataloging your book collection. Suppose you're testing a new code or looking at one in a PDF or on a website; wouldn't it be nice to confirm where it's going and what it's doing? Enter the $2.99 QR Right, available in the Mac App Store. QR Right, from Ripe Apps, behaves like a camera pointed at your screen, except without the camera.
When you run QR Right, you click the dock icon or the menubar icon to "scan" your Mac screen. Any detectable QR Codes or 1D barcodes are highlighted, with the decoded data right below them. If it's a URL, one more click will open that page in your default browser. QR Right is fast and accurate, and if you need the feature, $2.99 is a bargain. At the moment, the app is suffering from a cosmetic bug on the Retina MBP where it renders codes at 4x the correct size; that's not uncommon for apps that have to deal with the screen bitmap, and the disconnect between display elements and raw pixels is causing some issues for screenshot tools as well.
[Side tip: if you're a Retina MBP user, the indispensable €14 utility SwitchRes X should be on your shopping list. Not only does it give you granular control over all the Retina-friendly HiDPI screen configurations, it lets you force your screen into non-Retina versions of the standard resolutions when needed. Apps such as QR Right that are skittish with Retina behave just like they should when you dodge back to a straight 1440x900.]
The more "normal" side of the QR Code process would be the part where you hold a code up to a camera (or a camera up to a code) to read/act upon the content. That's where the free QRreader app comes into play. This free utility uses your Mac's iSight/FaceTime HD camera to scan printed codes; it promptly opens them in your default browser.
Unfortunately, since QRreader is built with Adobe's AIR framework, it's susceptible to outside factors that might break functionality; in this case, the latest version of AIR (3.3) doesn't allow QRreader to work properly. If you're still running v3 of AIR, though, it does the trick.
For making your own codes on the fly, there are plenty of free and premium online tools; if you'd like to integrate QR Codes into your database projects, however, I recommend the CNSBarcode plugin for FileMaker Pro. This $85 plugin enables instant creation of QR Codes (plus scores of others), and also enables barcode scanning within FileMaker. Codes can be calculated from any data fields, making it simple to programmatically generate codes for URLs, contacts and more. CNS Barcode also makes a $9.99 iOS app that can scan and create QR Codes and other codes, even allowing third-party apps to send URI requests for code creation.