Operating systems always have been my favourite computer related topic, so I know and use many: * '''Linux''' - I use or have used several LinuxDistros extensively for a long time. I occasionally do minor contributions to some of them. I've also put together and distributed a custom Linux distribution targeted at the AmigaComputer as part of the AmigaUnixCompendium project. (no, by Linux I don't mean GNU/Linux, I dislike this name even though I agree with the FreeSoftwareFoundation in general). * '''NetBsd''' - I've been using BSDs since 0.9. I even made a NetBSD distribution for the AmigaComputer called the AmigaUnixCompendium. See also OpenSourceDeveloperRole for some contributions I made. * '''Sun Solaris''' - I've been using it since Solaris 7, but a few years ago I've lost interest in it because of lack of development. Besides building hundreds of GNU and other packages to replace almost all of Sun's userland, I did port the early versions of KDE 1.x and 2.x to Solaris with quite some success. * '''MacOsX''' - I know this system quite well and have been using as a user and developer until mid-2006, when Apple started making some steps back from their previous OpenSource policy, showing lack of cooperation and disrespect for the OpenDarwin developers. * '''AmigaOs''' - I've been using and developing a lot on the Amiga in earlier '90s, mostly as a freelance developer. I expecially prised its system architecture and the OS architecture, therefore I used many of its best ideas in several EmbeddedDeveloperRole projects. * '''MicrosoftWindows''' - I've been using and developing for Windows with little enthusiasm since Windows 3.1 upto Windows 2000. I know most of the WIN32 APIs and concepts, including: GUI design with common controls and MFC, threading, asynchronous I/O, COM, database access, VisualBasic and VBA, DirectMedia and DirectDraw. But as of today, I'd recommend doing any new Windows development using portable tools such as Cygwin or MingW along with portability layers such as Qt or Gtk.