Mortgage paydown calculators? (loan, payments, interest, principal)

[totallyfrazzled]

I'm looking for an online "mortgage paydown calculator" to find out how much per month must be put additionally toward principal in order to make two mortgage scenarios equal, interest-wise.

Edit: Duh, I just found two online. If anyone else needs this info, the links are:

http://www.fiscalagents.com/toolbox/..._paydown.shtml

http://jrochelle.googlepages.com/ss-...t-paydown.html

[ricardocobos]

Here is a cool one that allows you to input additional payments beginning at different points in your amortization;

How advantageous are extra payments?