Over the last couple of months my pet project has been my online real estate calculator. This all stemmed from my initial work with my home price estimator. My original plan was to allow users to upload their own data via a CSV file. After a ton of work to ensure the software was general enough to accommodate all sorts of different data yet locked down enough to avoid SQL injection attacks I realized that having users upload their own data, in this case, was probably a pretty stupid idea. I have subsequently contacted the Northwest Montana Association of Realtors to find out if they license their data so we'll see where it goes from here. For the time being there is a demo page up with some real and some made up data. If nothing else it was a pretty cool exercise in incorporating some AJAX and especially using R to do all the calculations and make all the graphs in real time.
And as a random aside: As with a lot of the open source projects R doesn't have the pretty UI but holy cow is it powerful. The DBI/RMySQL packages made interfacing with our database almost trivial. Many thanks to the people that contribute, I just can't say enough good things about R.