Personal tool choices

As a user of lots of different software, and a great believer in using the right tool for a job, I am fairly picky about what I use. I have a few specific requirements:

• Open source - wherever possible I like to use open source software, partly because it is usually free (as in beer!), but also because it often comes with a good user base and readily available support. I'm very much taken by the idea that if I require a particular upgrade of alteration I can either email the developers directly or even make the change myself!
• Operating system independent - At home I use Ubuntu Linux as my primary OS. At University I'm (more or less) tied into Windows XP. On my sponsors laptop I'm very much locked into Windows 7 (and also very limited in terms of software choices - maybe I'll come back to that).
• Compatible - open source stuff is usually very good at this almost by default, but where I'm not able to use open source I need to be able to export and import stuff between programs.
• Portable - As mentioned above, I'm using a lot of different machines and networks, therefore I need to be able to easily move stuff around.

There's probably some other minor stuff as well about ease of use, etc, but you get the general idea.