Zen Cart: A full-featured, customizable e-commerce solution for businesses of any size. NOTE: This application has a steep learning curve, and will require several man-hours to configure correctly by the end user (you). While it is extremely powerful, and has built in integration with many online payment sites, it is not an application you will be able to use without serious effort on your part, but the result can be a world class online store.
Word Press: The premier blogging program, it is highly customizable with extensive instruction on line and many "skins" to allow you to rapidly modify the look of the entire package. It is so powerful, yet easy to understand, many people use it as their entire web site.
Media Wiki: The wiki used by many high end wiki applications, the best known of which is likely wikipedia. Daily Data uses this software for our technical notes at http://wiki.linuxservertech.com/ and our client information base at http://wiki.dailydata.net/. Fairly simple to use without extensive training, yet very powerful in allowing major customization.
PHPList: A highly customizable mailing list program that allows you to send newsletters, e-mail blasts and other types of e-mail to a group of people. You can have multiple mailing lists, and recipients can add/remove subscriptions via a simple web interface or by e-mail. Daily Data uses this list for contacting our clients and friends.
Gallery2 One of the best online photo albums currently available. With automatic thumbnail generation, and the ability to define albums and even individual pictures as viewable by limited groups or the entire public, Gallery2 gives you the ability to have a photo album customizable to your individual needs.
Drupal: Arguably the most powerful Content Management System available. Build and maintain your entire web site from one interface, with limited knowledge of HTML required for simple sites. Drupal is extensible and can handle everything from small, individual web sites to large community driven ones (which requires additional knowledge).
Firefox The best web browser on the market. High compatibility with the HTML Standards, plugins allowing greater customized capabilities, and works on all three major computer operating systems.
Open Office is a complete suite of full-featured office applications which competes directly with Microsoft Office.
GnuCash An excellent personal accounting package that can be run on Linux, OSX and Windows
The Gnu Image Manipulation Program (GIMP) Handles 99% of all your picture and graphics needs
phpGroupWare or eGroupWare: Groupware is a buzz word, but these two packages provide calendars, to do lists, address books, e-mail reading and writing, and many, many more functions all integrated in one package.
WebCalendar: Simple online calendar with ability to have multiple individual calendars, e-mail notification of appointments, and an administrative overview containing overlays of several different individual calendars. Very simple to learn and maintain, but not as powerful as some other applications. Used by Rod at Daily Data.
XRMS: A very advanced open source customer relationship management (CRM), Sales Force Automation (SFA) suite: also features business intelligence (BI) tools, Computer Telephony Integration (CTI), and advanced plugin architecture. PHP/ADOdb/LAMP (as per their web site)