There are some useful tools online that make the life of web developers alot easier. I have a few bookmarked that really come in handy. This varies from image tools, Reference and content.. Here they are:
Lorem Ipsum Generator for dummy text : http://www.lipsum.com/
Image Placeholders : http://placehold.it/
JSON Data converter : http://shancarter.com/data_converter/
Web Page Analysis : http://tools.pingdom.com/
Browser HTML5 Test : http://html5test.com/
CSS Sprite Generator : http://csssprites.com/