conjoon is a web-based open source application, built with the Zend Framework, that focuses on message exchanging.
Currently, it is a full-featured webmail & RSS reader client with contacts management. But the upcoming releases show that it will also have an integrated Twitter client with multiple accounts support.

Thanks to Ext JS, the application has a slick desktop-like interface with tabs, drag'n drops & more.
It comes with a web-based installer & requires PHP/MySQL to run.
Script.aculo.us is the most widely used add-on for the Prototype JavaScript Framework with the features like animations, drag'n drop, Ajax control & more.
Its second version, renamed as Scripty2, is out with a complete rewrite. It is lightweight (5kb compressed), flexible & works faster.

The library is currently in alpha status & there are still parts that need to be developed or improved. On the other hand, it is very promising & already being used by several big projects.
And, besides the Scripty2 homepage itself, the demos show how it will be empowering Prototype JS.
Retrospectiva is an open source project management & bug-tracking application that is focused on development teams.
It is built with Ruby on Rails & requires a MySQL database for storing the data.

It has 3 main modules:
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ticketing: to manage issues/requests
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code review / revision management: supports Subversion & Git)
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milestones / goals: to set & manage objectives for the project
And, comes with various add-ons like:
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wiki: manage documentation
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blog: to interact with the community, make announcements.
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agilePM: comprehensive support for your agile development process
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& more, which can be found in the wiki's installation pages.
Retrospectiva has an easy-to-use admin interface, a XML-REST-based API for integration with 3rd party applications & RSS support for following every content much simpler.
After The Deadline is a project of Automattic (the creator of Wordpress) which checks:
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spelling
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grammar
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styles (finds complex phrases, cliches, bias-language & suggests simple ones)
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and detects misused words (has a database of 1500 most misused words & suggestions)

The software used to come as a plugin for several projects (like Wordpress, RoundCube, PHP List) but it is now open source and anybody can embed it to their own code.
Besides the artificial intelligence it already has, it is possible to add new rules & improve the application.
The project also has libraries for jQuery, Ruby & Python to make developing new plugins easier.
Map Icons Collection is a set of 600+ free placemark icons that can be used to mention almost anything on maps with style.
The set is categorized very well in the project's wiki pages like restaurants, culture, sports & much more.
Sizes of the icons are mostly 27*27px or 32*37px and they are all transparent PNG format.

Using paper textures is always a popular trend in design of any kind.
Here is a set of 7 high quality paper & cardboard textures -created exclusively for WebResourcesDepot by TextureZine- that are free to use in personal/commercial projects.
All textures are in JPEG format & 3168*4752px size.
About the designer: Hakan Nural is a graphic designer, blogger & stock photographer. He is running
TextureZine which is a website presenting free & unique textures created by him.

Info: The download package is a .zip file & sized 46.2mb.

P.S. The textures can not be distributed in any other media. Please refer to this page if you mention them on your website. Thanks very much for that.
Every web application that works member-based needs a functionality where visitors can register & manage their profiles.
UserCake is a free user management system, built with PHP/MySQL, which aims to be a foundation to ease creating the application.

UserCake is object-oriented, cleanly written, well commented, formatted & documented.
It offers the following functions:
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Login
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Register
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Lost password
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Update password
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Update user details
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Email templates (optional)
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Account activation (optional)
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User groups (Basic, id – group_name)
The system comes with frequently used member-page functions like isUserLoggedIn() or isGroupMember($id) which will help developing faster.
And, in means of security, UserCake uses salt along with a SHA1 hash.
Floom is a MooTools slideshow plugin that can present images & their captions beautifully.
It displays them with a blind-effect & a chic loader that shows when the next image will be shown.

The plugin has various options like:
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defining the number of blinds between transitions
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slideshow interval
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loader on/off
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captions on/off
To mention, Floom is not a complicated script, rather, it is a lightweight (4kb compressed) one with very few features (just auto-slides images) which is sometimes "what is needed".
P.S. The download link in the website, currently, has a typo. You can use this address until it is fixed.
eCSStender is an open source JavaScript library, created by Aaron Gustafson, that helps browsers (including IE6) pay attention to the CSS code you write.
It aims to prevent the usage of browser-specific properties & hacks but make leaner & meaner CSS possible.

eCSStender is an extendible library with its API support & has ready-to-use extensions like CSS3 selectors, @font-face, rotation & more.
How does it work?
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it loops through the document's stylesheets and begins collecting information
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parses the rules it understands
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checks if the parsed rules are supported by the extensions
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applies the supported rules to the document via JavaScript
Read the rest of this entry »
XMail Server Lite is an open source & free mail server with an Ajaxed webmail bundled to it.
It is compatible with Window & Linux OSs with 2 different installers provided.

The server is capable of managing SMTP/ESMTP & POP3 protocols with web-based administration panel.
It supports unlimited domains/users including virtual domains & aliases.
XMail Lite also offers anti-virus integration, bayesian filters (for spam protection) & a logging functionality of all the activities.
The Webmail Lite can display folders/nested folders, has an address book, multilingual & much more.
Tip: The webmail can be downloaded & installed as a standalone product too (from here).