Yes, yes, I know (under Windows Phone 8.1) you can get to QR scanning through the 'Bing Vision' camera lens, but that's a bit of a roundabout way to come at the task. Better to have a dedicated application and even better if it's terrifically function-rich and also updated often. Enter QR Scanner+, which has just received a major update, with changelog below. It's the last update before a complete v3.0 rewrite too, which will be universal and also work on tablets and laptops.
QR Scanner+ scores for me because it's easy to kick off directly from the Start screen, but also because it intelligently retrieves the links and information you need on the fly, so you always get a preview of what you just scanned before going somewhere you didn't mean to go.
From the official QR Scanner+ changelog:
Barcode Orientation Fix
Header Buttons in AppBar (optional, see settings)
Added Result Specific Options
New Result Specific: Extended WP Store app Details
New Result Specific: Calendar, local timezone converter
Added all QR Scanner RS Settings to Rapid Scan Mode
Minor optimizations and BugFixes
NOTE: Version 2.2. is possibly the last update before QR Scanner+ 3.0. QR Scanner+ 3.0 will be developed from the ground up and will targeted as Universal App on Windows 8.1, Windows Phone 8.1 and Windows 10 (later).