There are a ton of them! I tried several, some free and some paid. I finally settled on HomeBudget...
http://itunes.apple.com/us/app/homebudget-with-sync/id306257910?mt=8&ls=1 ...There is a free lite version so you can try it out first. It is a bit confusing at first, when first trying to use it I got frustrated and tried yet another paid one but I went back to it and now only use it. It is feature rich which makes setting it up hard. Has nice graphs and other stuff. Cool thing with this is you can sync it with multiple devices. So if you have family members with iDevices that is a really nice feature. I use my iPhone for entering cash out and my iPad for managing the budgets and keeping track of all the info. It's $4.99/universal. I've emailed the developer twice with questions and got quick responses.
Edit:
I almost forgot to say
I can say this app has been useful for me, I have enough money saved for the new iPhone next month! Heh, I use iDevices to help me to buy new iDevices.