Hotel— Every hotel has their own Wi-Fi. Some hotels do have trouble with their network setup or the connection due to the historical location however at the least you will have free wi-fi at your hotel. All you have to do is to learn the wifi password to access the internet.
Cafes— Every café, bistro, restaurant share their internet with their guests. Even the small restaurants now have internet access. Stability and speed depend on where you are and what kind of café, bistro or restaurant you are in. Starbucks, Nero etc. typically have stable wi-fi unless very crowded. If you are in a Starbucks all you have to do is connect your device (SSID should be TTNET or DorukNet, AND if you are in Nero DorukNet) and fill out some basic information for verification that you have to fill. After that, you are ready to go. And if you are in the other restaurant or cafés you can just ask to your waiter to get SSID and Password and after that you are ready to go.
Public Center and Squares— Municipality of Istanbul recently announced that free public wi-fi will be available in most common city centers and squares. All you have to do is (when you near of one of these centers of course) register your id via your cell phone and you will get an access password.
Wi-Fi on the Go— You can rent a mobile wifi hotspot, ready local mobile phone packages (unlimited local calls, sms messaging and internet) and also international mobile phone packages during your stay in Turkey. It works based on 3G connection in the whole country, and you can connect up to 10 devices at the same time. These pocket-sized devices can be easily booked online.
While there are plenty of international companies that rent a mobile hotspot, mainly two local companies are operating: