Just came back from holiday and remembered that the "Super Sexy" Mortgage Calculator Contest was coming to a close. So I quickly knocked up an entry.
Check it out at http://igrapher.com/iMortgage.
There's lot's of features I'd still like to implement, but for 15 hours work over 3 days, I'm content with the outcome. Positive things about this calculator is that it doesn't use flash (so the charts work on mobiles), and does all the calculations asynchronously client side, so there's no wasted server interactions.