I recently simplified and starting using Google Forms.

I created a form with name (text), amount (number) and category (dropdown), then opened it in my iPhone's browser and added it to my homescreen. Whenever I make a purchase, I tap the icon and up pops a blank form. Takes me no more than 10 seconds to fill out.

You can link the form responses to a spreadsheet, and from there I can categorize and aggregate with this raw data any way I want. Next step is to create some graphs on the spreadsheet and add that view to my homescreen, so I can see how much budget is remaining in a month.

I've grown tired of budgeting tools that try to link your credit cards, etc--authentication maintenance is never the smoothest, and things are more detailed than I need them to be. Best thing about this method is that I have the raw data to work on, and I can adjust my data visuals easily as my needs change (it might also be cool to one day do large aggregates over, say, 10 years of personal spending)