With Gastus you can manage your expenses with concepts and several types. I’ve understanded that concepts are a kind of tags. Types are a kind of categories. I made it anyway like this.
Gastus shows you your expenses in several diagrams and flipcharts. In fact, there are two of this graphics. A cookie diagramm and a line diagram 
I’m testing it for this month. I think its a useful web 2.0 application to manage your expenses. It displays you how many money you expensed to which kind of category. Try it!
An API exists also for Gastus. I spent a little look to the API. Check out which functions are available!
Update:
Gastus supports now Apple’s iPhone! Take a tour.