I'm a big fan of the google charts API - it draws much better-looking graphs than I would ever manage and all I have to do is assemble the right URL to make it work. I recently got a feature request to add QR codes to joind.in, so that speakers and event admins could easily allow people to link in to a particular talk page.
To cut a long story short, joind.in already has short urls for particular talks, just in the form http://joind.in/[talk_id] so I created the QR codes to point to these. The URL looks like this:
http://chart.apis.google.com/chart?chs=400x400&cht=qr&chl=http%3A%2F%2Fjoind.in%2F%2F3338
The parameters, in turn, are the chart size, the chart type, and the URL to link to.
Absolutely simple when you know how - thanks Google Charts :)