I use my debit card for almost all my purchases, and for a while I have depended on my bank's website to keep up with my purchases and how much money I have. This usually works fairly well, but sometimes when purchases take a long time to go through it has really bit me in the ass. I detest writing on paper, so I devised a way to quickly enter changes in my budget to prevent surprises in my account balance.
It's really simple and not too pretty, but it works quickly and effectively and has saved me a lot of hassle already. Feel free to comment/join the project, its one of my first android apps and the first thing I've ever written that I've open sourced.