I've used different javascript libraries in different projects and realized to prepare a list of commonly used javascript/jQuery plugins every web developer should know. In this post, you'll find the list which might be valuable for your upcoming projects.

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25 Free Online Tools for Web Design and Development

Top 22 Firefox Add-ons for Web Designers & Developers

1. Autogrow - auto-growing textareas(as the user types it will expand vertically to accommodate the size of the entry).
2. BlockUI - simulate synchronous behavior when using AJAX, without locking the browser(generally use for pop up, dialog).
3. ckEditor - rich text and WYSIWYG editor.
4. ClueTip - show a fancy tooltip when the user's mouse hovers over (or, optionally, clicks on) any element.
5. ContextMenu - to create context (right-click) menus.
6. Downloadify - download of text files without server interaction.
7. Flot chart - produces graphical plots of arbitrary datasets on-the-fly client-side having interactive features like zooming and mouse tracking.
8. HoverIntent - To delay or prevent the accidental firing of animations or ajax calls.
9. Infinite Scroll - pre-fetching content from a subsequent page and adding it directly to the user’s current page similar to Facebook.
10. jEditable - inplace editor plugin for jQuery.
11. jHtmlArea - WYSIWYG HTML Editor for jQuery(lightweight and easily customizable).
12. jqGrid - for representing and manipulating tabular data (client side grid).
13. jqPlot - plotting and charting plugin.
14. jQuery Cookie - lightweight jQuery plugin for reading, writing and deleting cookies.
15. jQuery Form - jQuery Form Plugin allows you to easily and unobtrusively upgrade HTML forms to use AJAX.
16. jQuery UI - to build highly interactive web applications with rich effects, animations, widgets..etc.
17. jsTree - cross browser tree component.
18. Masked Input - allows a user to more easily enter fixed width and format input.
19. Multiple File Upload - select multiple files for uploading quickly.
20. MultiSelect - Multi Select Dropdowns.
21. PrettyPhoto - lightbox clone. Not only supports images, but also for videos, flash, YouTube, iframes and ajax.
22. ptTimeSelect - To select Time from timepicker.
23. ScrollTo - Easy element scrolling using jQuery.
24. Turnjs - Flip book (Page Curl) effect.
25. uploadify - to upload multiple files.
26. Validation - simple client-side form validation.
27. Modernizr - Taking advantage of the new capabilities of HTML5 and CSS3 in older browsers.
28. excanvas - HTML5 Canvas for Internet Explorer.
29. html5shiv - to enable use of HTML5 sectioning elements in legacy Internet Explorer.
30. IE9.js - make Microsoft Internet Explorer behave like a standards-compliant browser.

Share your opinion or suggestion and let us know about other must know plugin(s) you think should be included.