There are many money and expense tracking apps out there for iPhone, unfortunately many of them are ugly and very cumbersome. If you're looking for something really simple and fast, Next is probably your best bet.
Next is a very simple expense tracking app which simply lets you hit a category and put in your expense. It doesn't require additional info such as what the product was or where you spent it. It's made so that you're not made to input that info, it's just supposed to summarise your expenses for particular categories.
When you first open the app you're given various preset categories as you can see from the screenshot above. Unfortunately you can't add more categories but I'm sure the preset ones will have you covered. You'll also notice how some of the category buttons are a darker blue than others, the ones which are darker are the ones where most of your expenses are. So most of my expenses are on computers/gadgets and fuel.
You can swipe to the left to find expenses for the week, month and year. When tapping on a category you'll be shown a graph, this graph will show the days where you spent most.
You can swipe to the right to find more detailed info for each day and month. You can see exact amounts spent on particular days in a month. Months are across the top and then you swipe up and down to go through the days of that month. You can delete expenses from here by simply swiping to the left over an expense, you can even delete a whole day of expenses by doing the same swipe gesture over a date.
There was one very annoying problem I had with the app and that was finding out how to delete an expense. It is not obvious at all how to and it does not tell you anywhere how to do it. It took me a good 5 minutes of just tapping and swiping everywhere to learn how to delete an expense. This is a great example of where gestures need to be shown at the start up of an app, users can't just find them. It weren't even listed in the help menu how to delete an expense.
Other than that annoying hiccup the idea and concept of the app is brilliant. I'll actually be using it from now on to track my expenses simply because it's so quick adding expenses.