MySQL

A very good RDBMS (Relational Database Management System) that is free and works well. I have used MySQL for many things over many years and it always performed like a champ. There are a large number of tools and APIs and the documentation is very good.

PostgreSQL

I have not had reason to use PostgreSQL much, but the times I did it worked well. This is also a true RDBMS.

JBoss

The application server is free, works well, and is very active. Many sites use it, including mine for the Lexicon (among other things). The popular Tomcat container is part of JBoss now. RedHat purchased JBoss back in '06.

Glassfish

Sun has open-sourced their application server and made it available for free. It includes several components and is undergoing very fast development.

OpenOffice

OpenOffice provides a very nice office package with word processor, spreadsheet, presentation, and drawing functionality.

KOffice

(nyt)

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.

Linux

The Linux OS is so good it's amazing it is free. There are several distributions that provide nicely packaged software that works well together. My personal favourite distribution is Fedora, but Ubuntu also looks good. I have used several others including Debian and Slackware. Unless you know what you are doing stick with either Fedora or Ubuntu, both are excellent.

BSD

There are several different BSDs: OpenBSD, FreeBSD

Solaris

Sun recently released Solaris 10 as a freely available Operating system.