One of the things I find it useful for is copying links from PC to phone.
Being lazy it began with adding http://zxing.appspot.com/generator/ as a Web Panel in Opera so to always have an QR code generator accessible at all times.
But then that became too much of an effort.
So today I've created a bookmarklet to generate a QR code for the page being viewed that can be launched with only one click.
Simply create a bookmark on your toolbar with the following code rather than an URL so it can be open up a QR code when required.
I'm using it on Android but there are QR Code readers for Android, iPhone, Symbian and most J2ME camera phones.