It's really nice when comparing quotes for buying down a rate, or looking at the impact of additional payments. It's also incredibly easy to throw together. For anyone interested:

• payment = PMT(mortage rate/12, mortgage term * 12, -initial principle)
• interest = (mortgage rate/12) * remaining principle
• principle payment = payment - interest
• new principle = prev principle - principle payment

Throw that into a spreadsheet and drag down. Add a SUM() to figure out total interest paid or whatever. I have a VLOOKUP() to calculate interest for the current year for tax planning purposes, for example.

Maybe I'll start an unofficial weekly post about various calculations I have in my spreadsheet. I have something like 30 tabs, so surely something there is interesting to others.