Apple provides a nice “Smart App Banner” hook for developers to promote their iOS apps from within their web apps. Unfortunately Google doesn’t have anything like this for the Play Store. Now that we have Basecamp for Android, we want to promote it to customers using Basecamp on their phone browsers.
Thanks to GitHub there are a few nice solutions:

These combine both iOS and Google Play designs which we didn’t really need. In the end we rolled our own solution, but I based the banner design on Vitaly Glibin’s Smart App Banner.

I uploaded a working version of the HTML to GitHub. Please feel free to use this design to promote your apps in Google Play. Thanks again to Vitaly’s Smart App Banner for the inspiration.
And if you’re an Android user get the Basecamp app on Google Play!