WEBSITES

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

ARTICLES

Cool Istanbul (external link) : Detailed article on Istanbul.
Wiki Travel – Istanbul (external link) : Summary overview article on Istanbul.

BLOGS & PERSONAL SITES & OTHERS

Istanbul-pedia blog (external link): English blog/website about various topics on Istanbul.
living_Istanbul (external link)
Locally Pera (external link): English blog about Taksim/Beyoglu area.