When you are scheduling a shoot ideally what you want to do is know how the home is sited, i.e. which direction it is facing and what the sun angle is going to be when you show up because unless you are careful, you could be shooting directly into the sun and find it impossible to get an acceptable front exterior shot.
This is exactly what LightTrac will do for you. You give LightTrac a address and it shows the sun angle and elevation at sun rise and sunset as well as the sun angle and elevation at any given date and time. If you have this app available when you are making an appointment for a shoot if you have this app you can be sure you are not going to have a sun problem or while the client is on the phone you can move the shoot to a time when the sun is at a good angle. I've shot listings I had to go back 2 or three times because I didn't have this app.
This app is hands down the best app I've seen for this purpose! The same app works on both the iPhone and iPad. You can easily get by on an iPhone screen but using it on the iPad is a perfect size. Click on the image above to see a full size iPad image from my iPad screen.
Thanks again to John and Steve! This is an awesome app... well worth $4.99 $9.99 it actually went up in price since I wrote this post- honest.
Update 10-09-2010: See Chris's comment below. This app is very similar to The Photographer's Ephemeris by Stephen Trainor, a donation supported desktop app for Windows, Mac & Linux. The Photographer's Ephemeris (called TPE) has a $8.99 iPhone app.
Second update 10-11-2010: Since I wrote this post Lighttrac has gone up in price. Also, I got TPE to compare it to Lighttrac. If you are going to use it just on an iPhone Lighttrac and TPE are pretty close but if you have an iPad Lighttrac works on either and on the iPad it really shines because it supports the large screen. Also I personally like the time of day slider on Lighttrac.