Amy and I are preparing to buy a house. We have enough of a down payment that we can make a purchase (although not as much as we want) but right now we're looking towards finding the right house and the right mortgage deal.
As part of the planning process I used an Excel spreadsheet to examine my options. I like the hard numbers that the spreadsheet gives me. If you're more of a pictures person, then a graphical tool can also a be found on the web.
Since we aren't going to have a 20% downpayment, I'm spending most of my time weighing different downpayment sizes and the impact of mortgage insurance vs. a second loan. Now that mortgage insurance is tax deductible, it is giving the second loan a good run for its money.
UPDATE: Private Mortgage Insurance (PMI) is treated as a form of interest as of Jan 1, 2007. This means PMI is now tax deductable on 2007 returns, but it is not deductable on 2006 returns. Unfortunately this change in tax rules also expires at the end of 2007, so there is no knowing whether it will be deductable in 2008 or not.