Excel.
I've got all my account balances projected out for the next two years. It is as detailed as to have a running tally of utility bills, average for each month, and projections of utility rate increases so that my monthly account estimates are as accurate as possible. It is quite useful. Tells me that at my current spending December will be a very tight month, but then I'm good for as far as I can project. Heck I even know what day in December I have to be worried about having enough in the checking account to pay a bill.
But then, numbers amuse me. So this is a fun hobby.