BondBuster allows you to monitor your mortgage bond in terms of balance outstanding, monthly payments, and interest paid per month or per day. You can easily modify parameters for scenario testing.
Data is stored in simple xml files so it can be kept up to date with your particular situation (ie. changes in payments & interest rates). It also has a handy graph which illustrates the portion of interest versus capital paid per annum.
Requires Java Runtime Environment (JRE) 1.7 or above. Get It Here.