Looking for a good plugin for donations like the one at truthdig

If you go to truthdig dot com you will be confronted by a “Donate” popup. If you click it, it will bring you to a page of different levels of donation.

What would be the best way to do something like this on a wordpress site with plugins?

I want to have a popup like that, that only comes up once for the viewer (or maybe once every week or something).

Any suggestions?