I use the following open source software tools ...

I use Open Source Software where ever i can. Simply because there is so much cool software out there. Free software enables me as developer to stand on the shoulders of giants. By simply adding some glue code to existing solutions you get individual solutions.

Office software

Openoffice of course, on the Mac comming via NeoOffice 

Software Development

MySQL, Postgresql, SQLite  as Databases

Python as Programming language,

Scite , vim  or eclipse as Editors  

Zope, Django or Turbogears for Webdevelopment

Bazaar as DVMS 

Graphics

Blender is great, the Gimp essential (on Mac Seashore) and Nodebox does wonderfull things.

Operating Systems

Well, as i have an Apple mac book pro 17" i can use all major operation systems (even at the same time :-).

• I work with Mac OS X of course;

• still my current favourite operating system is Ubuntu 7.04 which is simply great, the 3D GUI rocks, it is stable, comes with a giant amount of easy to install software and is for FREE!

• Last and least: for testing purpose i have also got Vi$ta. Some still use window$, so i needed to test my software against it; but can not recommand it.