Taking on a mortgage is one of the biggest things you'll ever do in your life. So it pays, quite literally, to know exactly what you're taking on.
Mortgage Calculator allows you to calculate conventional and interest-only mortgages, supports adjustable rate mortgages and provides information on annual percentage rates with respect to the service payments you'll have to make.
You can look at any year and see exactly what the mortgage and allows you to see detailed per-month information (rate, principal, interest, extra payments, total per year and remaining balance), as well as change interest rates and specify projected extra payments that will affect the loan total.
The problem with this program is that it can't calculate mortgages based on special mortgage deals specific to the bank you're thinking of going with.
So far example, it can't really deal with the fluctuating value of the Euribor for users in Europe. Nevertheless, it can give you a good estimate of what the coming years will cost you but don't rely on it.