What Open Source Software Do You Use?

Open Source Software packages are developed by a community. Anyone can see the computer code, suggest improvements, request features or report bugs.
The software is generally made available for free download.

This makes it very suitable for start-up businesses!

However, don't make the mistake of thinking of it as amateur software! Some of the world's biggest computer companies (Like Sun) are involved, as well as lots of individual professional developers. The software is normally of a very high standard.

Wikipedia has an article on Open Source Software.

Also, I suggest in Week 1 of Start Your Business Week by Week that people can use open source software in their startup. I recommend:
OpenOffice.org
Scribus
Drupal
because they are programs that I use.

What open source software have you used? Did you like it? Did it save you money? Please post your suggestions and reviews below....