Go Turkey : Official travel and tourist guide of Turkey
Turkey Travel Planner : Vast detailed website on Turkey and Turkish culture with a large section dedicated to Istanbul.
Istanbul Trails Dedicated large website about everything on Istanbul.
istanbul.com: English website dedicated to Istanbul, containing a large catalogue of articles on various locations and venues.
I was in Turkey: English website with blogs/wiki/forums about travelling in Turkey.
Timeout Istanbul : English Timeout articles and guides on Istanbul.
Istanbul Trails : Nice blog/website on Istanbul with a wealth of information on places, tips, culture, and history.
NY Times Istanbul : Section dedicated to travel in Istanbul, hotels, eateries and things to do.
Istanbul City Guide : Has a large searchable listing of venues/events, fair and hotels.
Mynet Fesmekan : Comprehensive listing of venues/locations in Istanbul (and other cities) in Turkish.