from zencart.com:

Zen Cart™ truly is the art of e-commerce; a free, user-friendly, open source shopping cart system. The software is being developed by a group of like-minded shop owners, programmers, designers, and consultants that think e-commerce could be and should be done differently.

Some "solutions" seem to be complicated programming exercises instead of responding to users' needs, Zen Cart™ puts the merchants and shoppers requirements first. Similarly, other programs are nearly impossible to install and use without an IT degree, Zen Cart™ can be installed and set-up by anyone with the most basic web site building and computer skills.

There are hundreds of shopping cart programs available, but none come close to offering to level of options, features and support available with Zen Cart™. Even the commercial programs have a hard time competing.

$200
Installation, Initial Setup, and Template Installation
Price Includes 1 year of hosting and a free domain

from osCommerce.org:

osCommerce has attracted a large growing e-commerce community that consists of over 177,500 store owners and developers who support each other and extend osCommerce Online Merchant with add-ons being contributed on a daily basis. To date there are over 4,700 add-ons that are available for free to customize osCommerce Online Merchant online stores and to help increase sales.

osCommerce Online Merchant is an Open Source online shop e-commerce solution that is available for free under the GNU General Public License. It features a rich set of out-of-the-box online shopping cart functionality that allows store owners to setup, run, and maintain online stores with minimum effort and with no costs, fees, or limitations involved.

With over 8 years of operation, osCommerce has built a showcase of over 13,600 online shops that have been voluntarily added to the live shops section, and powers many thousands of more online shops worldwide.

$200
Installation, Initial Setup, and Template Installation
Price Includes 1 year of hosting and a free domain

from joomla.org:

Joomla! is one of the most powerful Open Source Content Management Systems on the planet. It is used all over the world for everything from simple websites to complex corporate applications. Joomla! is easy to install, simple to manage, and reliable.

The basic Joomla! package is designed to be easy to install, even for non-programmers. Most people have no trouble getting our software up and running, and there is plenty of support available for newbies. We have a growing, active community of more than 150,000 friendly users and developers on our forums eager to help.

Once Joomla! is installed and running, it is simple for even non-technical users to add or edit content, update images, and to manage the critical data that makes your company or organization go. Anybody with basic word processing skills can easily learn to manage a Joomla! site.

Via a simple, browser-based interface you will be able to easily add new press releases or news items, manage staff pages, job listings, product images, and create an unlimited amount of sections or content pages on your site.

$150
Installation, Initial Setup, and Template Installation
Price Includes 1 year of hosting and a free domain

from wordpress.org:

WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress is both free and priceless at the same time. More simply, WordPress is what you use when you want to work with your blogging software, not fight it.

We offer a feature set with WordPress on par or better than any other software of its kind. Also we are committed to making the latest blogging technology available to our users (such as Trackback) and taking it a step further (such as with Pingback). You can rest assured that with WordPress you will be on the cutting edge of the technology available.

$100
Installation, Initial Setup, and Template Installation
Price Includes 1 year of hosting and a free domain

from phpbb.com:

Since its creation in 2000, phpBB™ has become the most widely used Open Source forum solution. Like its predecessors, phpBB™ 3.0 “Olympus” has an easy to use administration panel and a user friendly installation process, which allows you to have a forum set up in minutes.

With a large and highly customisable set of key features coupled with a standards compliant interface, phpBB™ will grow with, and compliment your website. With millions of installations worldwide, it is no wonder phpBB™ is one of the most popular ways to create and support a community.

$100
Installation and Initial Setup
Price Includes 1 year of hosting and a free domain

from bambooinvoice.org:

BambooInvoice is free open-source invoicing software intended for small businesses and independent contractors. Our number one priorities are ease of use, user-interface, and beautiful code.

BambooInvoice was built by designer and programmer Derek Allard, who uses it everyday, not by a large firm who can't remember the names of its customers. It is meant to be sexy, both on top of, and under the hood. Go ahead, kick the tires. View source shows semantic, structured, meaningful XHTML. We use AJAX to keep things peppy and javascript to keep things functional. The entire application degrades gracefully for users without javascript enabled, and complies with WAI priority two.

$75
Installation and Initial Setup
Price Includes 1 year of hosting and a free domain

from osticket.com:

osTicket is a widely-used open source support ticket system. It seamlessly integrates inquiries created via email and web-based forms into a simple easy to use multi-user web interface. Easily manage, organize and archive all your support requests and responses in one place while providing your clients with accountability and responsiveness they deserve.

osTicket is an attractive alternative to higher-cost and complex customer support systems; simple, lightweight, reliable, open source, and easy to setup and use. The best part is, it's completely free.

$75
Installation and Initial Setup
Price Includes 1 year of hosting and a free domain