If anyone is planning to bring their iPhone (or other app-capable device) with us on the trip, here are two apps that might be a good idea to download!
This app allows you to send text messages over the internet instead of through your wireless provider. If you are worried about roaming charges and don't want to pay for an international plan, simply switch your phone to "Airplane Mode." (Settings panel on iPhone.) This will prevent you from using 3G, and will only allow you to use wireless networks nearby (i.e., the one at the hotel.) Then, when wireless internet is available, you will be able to send and receive "text messages" through What'sApp. Note: Only other What'sApp users will be able to talk to you, so make sure to tell your family/friends to download it before you leave.
Download for iPhone: https://itunes.apple.com/us/app/whatsapp-messenger/id310633997?mt=8
Download for other devices: http://www.whatsapp.com/
Triposo's Istanbul Guide
This app is a nice little guidebook for the city. Most of the information is available offline, so you don't have to worry about roaming charges. It features a map of the city, a fairly comprehensive phrasebook, and information about places to see, eat, shop, and more!
Download for iPhone: https://itunes.apple.com/us/app/istanbul-travel-guide-by-triposo/id430534193?mt=8
Download for Android: https://play.google.com/store/apps/details?id=com.triposo.droidguide.istanbul&hl=en