Check if your page is visible currently via javascript

Link here.

Learn how to use flexbox to position elements

Link here.

Extract and manipulate rar files using javascript !

Link here.

Really cool concept of automatically switching to lighter or darker version of an element depending on the brightness of images behind it.

Link here.

A quick 3 minutes introduction to Bower( Javascript Package Manager)

Link here.

A nice and minimalistic progress bar

Link here.

Coolest ever page transitions !

Link here.

Preload assets(eg. images) on your page

Link here.

Know which css is going unused

Link here.

Tired of figuring out how to vertically(or horizontally) center your elements? Here's a javascript plugin!

Link here.

Get placeholder images according to your need when designing a new web app

I could find not one but three of them. Besides choosing sizes, we can also choose type of images we want from categories provided, all for free.

Lorem Pixel
Fakeimg
Placeimg

Easy library to make charts and graphs

Link here.

Want to minimize your minimized files more through structural optimization?

Link here.

A command line tool to generate css sprites.

Link here.

Sencha gives you a free way to resize your big images on smaller screens.

Link here.

Count character, words and paragraphs entered by a user live

A very small plugin that has no dependencies.
Link here.

A plugin if you want to use excel/spreadsheet formulas.

Link here.

Ready css3 animations

Link here.

Know the resolution and ppi of any screen

Link here.

Write better html using HTML inspector plugin

Link here.

Beautiful analysis and visualization tool

Link here.

A javascript library, Cytoscape.js for analysis and visualization

Link here.

Simple triangular list bullets using css

Link here.

Check user's input without js using regular expressions and HTML5 !

HTML5 now allows you to check input attributes of the user using regular expressions without the use of any javascript using "pattern" attribute -

A simple example for email check -

<input type="email" pattern="[^ @]*@[^ @]*" value="">

A font for weather forecast

Link here.

Modify select box with a much better ui with minimalect

Link here.

Need more visible area when you type in a textbox?

Link here.

Make background colors look more realistic without photoshop

Link here.

Simple nice plugin to show tips on hovering on elements the way you want to.

Link here.

A checklist of What all to check before starting user testing

Link here.

Learn how to efficiently handle garbage collection when working on an HTML5 game

Link here.

A javascript library to enable gamepads and joysticks in the browser.

Link here.

A nice and responsive gallery for your photos

Link here.

A collection of legal contract documents

Link here.

Super clean tool to animate your elements through css

Link here.

Responsive datepicker

Link here.

Remove debugging statements while moving code to production

Link here.

Library to build sql query strings on nodejs server side

Link here.

A lightweight responsive framework that combines the good features of bootstrap and foundation

Link here.

Create your own custom jquery builds with grunt

Link here.

Load high resolution image for retina display screens

Link here.

Swipe book pages with hand gestures with the help of your webcam!

Link here.
Try it once. It's simple and awesome!

A preprocessing language for css that works on javascript

Link here.

Reducing number of malicious url hits using 5G Blacklist

Link here.

Review your website on different screen resolutions on nodejs

Link here.

Node server for dynamic, fake JSON

Link here.

50 Javascript libraries for charts and graphs

Link here.

Want to tell your users to update their browser version?

Link here.

Awesome text animations

Link here.

Looking to make a jquery plugin?

Lots of jquery plugin patterns to start off here.

A small fun experiment on audio input.

Link here.
(Works only in chrome as of now).

Want to check if gzip compression is working on all files you want on your website?

Link here.

Readymade plugin for Paypal 'Buy now!'

Link here.

Match your selectors using substrings in CSS3

Link here.

One stop for all nodejs needs

Link here.

Create your own analytics dashboard

Link here.

Cool patterns and background through just css

Link here.

Super cool windows UI type animation

Link here.

Create tooltip type of toolbars!

Link here.

Tutorial to create an animation on canvas using image sprites.

Link here.

Awesome and free Slideshow plugin!

Link here.
It's responsive too! 

Get a hold on File API

Link here.

Create your own custom font icons using command line from svg images

Link here.

Watch the changes in any object or attribute

Link here.

Content security policy explained

HTML5 Rocks has a complete article on content security policy to save us from cross site scripting!

Check broken links on your website

Link here.

Detect timezone using javascript

Link here.

Tips on designing a credit card payment form

Link here.

Clean up messy html/css/js code

Link here.

Customise bootstrap theme

Link here.

Expanding the overlay div beautifully

Link here.

Remote preview

Get a remote preview of your mobile site on different sized devices here.

Polyfills for incapable browsers ( Webshims Lib )

Best thing about this plugin is that it only downloads the features that are not supported by a browser. That means, the more features supported by a browser, more lightweight the plugin gets.
Link here

Learn sprites use quickly with a cool demo!

Link here.

A super cool way to critique the webpage design by zurb!

Link here.

Awesome static file server for Nodejs

Lactate serves as a static file server for nodejs and is currently faster and better than all others out there.
Link here.

Searching for online android emlators ?

Link here.
If you're working on your android app and you don't have all the devices to test it on, here is an answer.
You can run your app on online emulator provided for almost all sizes and resolutions of android devices on this online service.
No hassle to download/install any SDKs or other emulator setups.
It's in beta still and is absolutely free. 

Tutorial on knockoutjs

Link here.

A javascript validator for nodejs

Link here.

A list of jquery chat plugins

Link here.