Nifty CSS animation and everything. The text that shows up in the popup can be customized in your config, and you can pass custom strings when using the tag in a post. It currently creates markup for a PayPal link and required hidden fields, but it could be easily modified to work with another system.
I’m not going to write a lot about this because it pertains only to Jekyll/Octopress users, and will only be of interest to a few of them. All of the details are in the repo. Of course, the HTML and CSS2 are in the repo, too, if you just want to see that.
See the whole shebang on GitHub.