1. Ubuntu/Windows XP/Mac OS X
2. Firefox (+extensions)
3. Azureus
4. Amarok/iTunes
5. VLC
6. Skype

and optionally

1. Thunderbird+Lightning/Mail+iCal
2. Gaim/Adium
3. F-Spot/Picasa/iPhoto

Ok, decent media support in Firefox on Linux would be nice too, but this'll do for now.