Teambox is an open source platform that enables teams to collaborate on projects with a Twitter-like interface.

The application is built with Ruby on Rails and, besides the open source version, has a hosted version too.

Users can create an unlimited number of projects & tasks, assign them to their colleagues, share messages, files & much more.

Users can start a conversation instantly, create pages to store information & Teambox can integrate clients to the system to colaborate with them easily.

The application is multilingual, has support for mobile phones & offers a read-only API for integrations with 3rd party softwares.

MyClientBase is a web-based, free & open source invoice management application which is built with PHP (using the CodeIgniter framework) & MySQL.

It enables anyone to create invoices, view payments, get reports & more..

The application is flexible & variables like:

• currency symbol
• date format
• next Invoice ID
• tax rate

can be customized easily.

It is possible to define tax rates per invoice or globally & create multiple invoice templates.

W-Script is an open source application which is originally aimed at creating "wallpaper download websites".

But, it is a very ideal resource for building any gallery-themed or image-oriented website like CSS galleries, photo galleries ,etc.

The application is built with PHP-MySQL & it can handle large number of images.

Some features of W-Script:

• unlimited categories, subcategories
• visitors can submit images (and optionally register)
• images can be rated & commented
• advanced search
• auto-thumbnails in various sizes

It has a Smarty-based template engine which makes creating new templates pretty easy.

Also, W-Script can be extended with modules like tag clouds, color browser, user stats or anything custom.

DHX Quick Tables is a 1 file PHP script that enables anyone to create a customized backend for MySQL databases with 2 screens:

• for guests: just view the tables
• for administrator: view, edit, add/delete pages

The script comes with a web-based installer that guides you to define the database connection information.

After that, you can customize the fields of the database by selecting if they can be sorted, edited, in which order they will be displayed & more.

DHX Quick Tables is very functional who wants to build an instant backend for their applications or create an interface that will display a tabular data.

Symphony is a XSLT-powered open source CMS that is created with PHP & uses MySQL for storing data.

It offers a very flexible usage as the data, logic & templating layers are all independent, meaning that whatever you implement can be modified, added or removed with minimum effort.

When compared to other popular CMS applications, the biggest difference is the template engine used. It is XSLT-based which makes it possible to output much more than just web pages (dynamic CSS & JavaScript, PDFs, SVGs..).

Symphony has a light & elegant backend that helps you focus on your work with features like bulk editing, drag'n drop ordering of items & much more.

The application has a growing library of extensions (130+) which can help you improve the functionality it offers.

Crystal is a database toolkit for PHP which aims to help developers build & scale reusable, database-dependent, framework-agnostic applications.

It has 3 independent components:

• Database wrapper – for CRUD operations
• Validation module – provides a comprehensive data validation
• Database manipulation module – used for creating databases, tables, etc.

The queries written with Crystal are database-independent which currently supports MySQL & PostgreSQL (SQLite is on the way).

For anyone who is familiar with SQL, learning Crystal wouldn't take no more than 5 minutes.

And, for many developers, it can be an option against framework-specific database layers like Zend_Db and Codeigniter's active record library or complex ORM's like Doctrine.

rsslounge is an open source, full-featured & web-based RSS feed reader application that is built with PHP-MySQL.

It allows you to read your feeds, categorize or filter them, set priorities & more from an Ajax-based slick interface.

Besides the standard feed items, rsslounge supports images & photoblogs.

For periodically fetching content from feeds, it offers 2 options: cron job or Ajax-based requests (that work while the application is running).

The application makes switching feed readers so easy with the OPML export/import functionality.

Also, rsslounge is a guide-like application for jQuery fans as it makes use of this library fully with the Ajax requests, accordions, sliders, modal boxes & more..

P.S. It works perfectly in all major browsers except IE.

IntraMessenger is a free application for that enables communication between communities, groups, companies or simply closed groups.

It consists of a client (for Windows) & a PHP-MySQL-based open source server to manage the whole communication.

The messenger is autonomous, but authentication (login + password) is possible from other existing databases like forums, groupware applications, e-learning systems, etc.

It can operate in 2 modes:

• open community : everybody can see everybody, without add to contact list
• groups only : everybody can see everybody (but only) is they are in the same group

IntraMessenger has a multilingual interface & it is a handy application for enabling web-based communities to communicate in the desktop.

nuBuilder is an open source web application for quickly creating web-based back-ends.

It allows you to create forms for inserting-updating data & generates reports with that data.

The back-ends are created with PHP/JavaScript & the data is stored in a MySQL database.

nuBuilder can be used to create complex applications like accounting systems or simple lists like customer phone numbers.

It  has built-in components like permissions, search screens & data validation that makes the development process faster.

The application can be expanded with modules & plugins.

nuBuilder is also a handy application for web developers who create a web-app & want to put it online as quickly as possible without spending time on building the back-end for it (at least for a while).

GaleriaSHQIP is a free PHP-MySQL image gallery script for easily displaying any number of images.

It has an admin panel to upload new images or delete/update the previous ones.

The script offers a simple interface for presenting images & upon-click, they can be viewed in their original sizes in a modal box (uses piroBox).

GaleriaSHQIP has a web-based installer for a quicker installation & has a built-in brute-force filter + various other security features.

Update (8th February 2010):

The application now has a new version with lots of features like tracking visitors/their actions, commenting, rating & more..