Tipsy helps you make donation to web site whose content you read. You specify an overall donation amount. Tipsy tracks your use of web sites over time (that information stays with you; nobody else can access it) and periodically divides up your donation among those sites proportionally to the time you spend on them. You have complete control---you can change the amount you donate to each site or choose not to donate to particular sites. You can make the donations whenever you want.
More information is on the project web site, at http://tipsy.csail.mit.edu/