For some reason it won't run on the MMIS server, but I'm working on it. A few people have asked me about this one, so I researched it out and wrote it up. This simple JavaScript does a monthly compounding on a fixed balance as well as contributions. Here's a simple little JavaScript calculator that shows you the after tax interest rate of your mortgage. It also shows the after-tax return on your investment income.
Some people complain that their browsers crash when pulling down an amortization schedule.
You can either use a full schedule without any prepayment, or start with your current loan amount and enter any monthly payment amount you want. This uses the formulas and discussion from Stanley and Danko's best-seller The Millionaire Next Door so you can see how wealthy you really are. They are really too good to believe. Here is a simple JavaScript example for an interactive calculator. It just takes a few inputs and figures out your monthly payment.