I am constantly amazed by how much great software there is available for free.  As an introduction, here are some that I use on a regular basis:

• Apache web server 2.2
• GCC 3.4.5
• Java 1.6.0_18
• MinGW / MSYS
• Mozilla Firefox 3.5.8
• OpenOffice 3.2.0
• Python 2.5, 2.6, 3.0 and 3.1
• SciTE 2.01

I also use Opera 10.53, which is free but not open-source, and Windows XP / 7 the majority of the time – not free or open-source.  However, I prefer to write code that works across platforms as far as is possible.